Heritage is the best i've found...

There are a number of auction sites out there, and depending on the level that you collection has reached, you might consider Heritage Coins (heritagegalleries.com). i used to be hard core eBay (and still patrol those murky waters), but when i found heritage, i was hooked. first, they have an auction archive, which lists the prices paid for any given coin over the past 10 years--that alone is a superb asset. secondly, list the major list prices for the coin, including wholesale. third, (although this probably should have been first), they have a huge inventory and since auctions are always running, just about anything you could want will eventually appear. the coins are generally better looking, and i almost always pay less that what i have seen a similar coin going for on eBay--i think that might be because less collectors have found this site, and so the bidding wars one sees on eBay are generally not present.

it's free to join, and it shows the price with commission before you bid. i hope this helps (and maybe heritage will give me a discount for all these nice words).

good luck, and happy bidding.