A real gripe for me on here is selling without a guide price.
I also collect neo geo aes games but something which is used a lot is the price guide. http://www.neo-geo.com/guides/aesguide.html
Would it be possible to do something similar. Either updated manually or something that takes an average of ebay sales in the last few months.
This could then be a starting point to base prices on. It could take an ebay price and reduce it by 10% to reflect fees. You could also show a min and a max price sold as well as average.