I have spent many years purchasing bulk unsealed Lego lots off of Kijiji and I think I have some general advice to help out others doing the same or thinking about doing the same. As we all know, Lego can be very expensive if paying full retail price for new sets, while extremely good deals can be found if you are willing to spend a little time and a little effort.
One caveat: I have never sold even a single part and have no intentions of doing so. This advice is not for those looking to start a Bricklink store, make some pocket money, etc.
1) Think about what you want to collect before purchasing
Do you want to simply build up parts for the purpose of MOCing? Do you want to build official sets? Do you care about minifigures or not? Are you willing to use Bricklink to fill in those last remaining parts for sets? Do you want to focus on certain timeframes and/or certain themes? If you have strong opinions on any of these questions, let that guide your searches and purchases. For example, I prefer to collect sets from roughly 2000-onwards. When I see a bulk lot from the 1970's or 1980's, even if it is a good deal, I will skip it since it will likely not assist with commencing or completing sets I would be interested in (due to part selection, colours, etc.). However, don't take this too far. While you might not be a Technic fan, a bulk Technic lot is extremely useful as most recent sets contain Technic parts (e.g. pins, liftarms, axles) and those parts (especially the smaller ones) are sometimes missing from otherwise complete sets.
2) Learn to use Bricklink inventories and parts guides
I am surprised how often a bulk lot, especially one that was a child's collection being sold as that child is now a teenager or young adult moving on from the hobby, is completely broken apart. Obviously, a posted ad where every set is (mostly) assembled is easy to identify - the tough part is trying to gauge what sets may be included when every part is separated! I start with identifying unique parts and/or unique colours and using Bricklink to narrow the potential sets. At worst, you usually can identify the timeframe these parts came from (since it is rare to find a bulk lot spanning multiple decades).
3) Minifigures or not?
Not surprisingly, most lots identify as having no minifigures or limited minifigures. This is usually not due to a seller having advanced Lego selling knowledge or trying to sell minifigures separately but simply because minifigures get abused, lost, defaced, etc. way more often than regular parts. My collecting habits are to build complete sets including minifigures but I have bought more lots without minifigures than with in my life. Why? Because it can be so much cheaper! Other than certain exceptions, many minifigures can be bought from Bricklink to complete sets at way less cost than what a bulk lot with minifigures will go for.
4) Ask questions
I rarely buy a lot without asking questions first. This isn't like Storage Wars where you have two minutes to view a job lot and then bid blindly. If photos are blurry, ask if clearer ones can be taken. Ask if instruction manuals are included and ask for photos of those (although be aware that just because a manual comes with a lot, does not mean that set is also included). Ask about the presence or not of minifigures. Ask if the lot comes from a smoke and pet-free home (important if you want to avoid much cleaning [see below]).
5) Always view in person before purchasing
I'm sure others will contradict my advice on this one, but I simply refuse to buy bulk lots of eBay with just photos for support. Viewing the parts in person allows you to confirm the size, condition, cleanliness and accuracy of the listing. Many people will list "Lego" but the lot is mostly or partially other brands. For those who don't collect Bionicle or other constraction sets, a bulk lot of 40 pounds where 30 pounds is Bionicle would not be acceptable (which is very common amongst kids about to go to university and getting rid of their Lego as Bionicle would have been very popular during their peak childhood collecting years). Not to say everything is bad - I've had people guess at lot size (e.g. "around 20 pounds") and when I go to view in person the actual size just using my own knowledge is much larger.
Don't be put out by lightly dusty Lego. I would direct others to review prior forum posts about cleaning Lego for more discussion, but I find it quite easy to un-dust Lego. However, gunky sticky residue, the presence of glue or silly putty, etc. makes almost every lot not worth the price you are paying.
7) Sorting - by part type
There are many, many forum posts debating colour vs. part type sorting. I am not going to dive into that debate. However, if you are going to sort by part, having good storage solutions is essential; I find Tom Alphin's labels and storage advice to be the cream of the crop. I think if you are looking to MOC, this is the sorting method for you.
8) Sorting - by colour
If you are trying to build sets from your lots, sorting by colour can sometimes be more efficient than by part type. I use Bricklink inventories printed to OneNote on my tablet and since those inventories are listed by colour, I find it quicker using this method. I also find that some lots are already sorted by colour (it would be very rare to find a lot sorted by part unless a MOCer is selling their entire collection) and it would be inefficient to lose the work someone else already did for you.
9) Sorting - particular colours
As some of you may know, there are several colours which are very difficult to tell apart. While I find Dark Gray/Dark Bluish Gray to be rather simple to tell apart, Light Gray/Light Bluish Gray is very hard especially when it comes to Technic pins and bushes. Brown/Reddish Brown is also somewhat tricky and Pearl Light Gray/Flat Silver is even tougher. Having sample 2x4 or larger bricks of each colour on a baseplate is very helpful.
10) Sorting - Axles
I can't find the link right now, but someone has posted a very helpful guide to print on a single sheet of paper which has the various sizes of liftarms and axles to compare against as differentiating between an Axle 5 and an Axle 6 is difficult for even the most seasoned collectors.
11) Sorting - miscellaneous
- I find it helpful to separate printed/stickered parts from all others as this is usually an excellent way to narrow down what sets a part is from
- Same advice for unique parts. This takes some advanced knowledge but eventually you will come to know what parts are very common (e.g. 1x2 plate), what parts are somewhat common (e.g. round 1x1 brick), what parts are somewhat rare (e.g. modified 1x4 brick log) and what parts are very rare (e.g. 6 x 8 x 9 tower roof). Picking out the rare and very rare parts will also speed up your search.
- Same advice for unique colours. This differs greatly depending on what era(s) and theme(s) are being collected, but in my experience with all themes from 2000-on, the least common colours that aren't complete one-offs are dark flesh, dark orange, bright light yellow, the various sand colours and some of the blues/pinks/purples that are increasingly more common from the Friends theme. More common but still highly desirable are dark red, dark brown, dark tan, olive green, dark green, dark blue and the various metallic and speckle colours.