ShopGoodWill.com - 3 shipping is always too high plus most add an extra 15% extra you have to pay
http://www.japanvideogames.com/ - 3 service and shipping is great but pricing is terrible
https://www.risingstuff.com/ - 4 pricing is great, shipping a tiny bit higher than others and its only run by two guys so updates and orders are slow. But they will help you out and find stuff for you also.
Racketboy - 5 what can I say bad not one bad experience
Neo-Geo - 4 I find plenty of great sales on this site and haven't had a problem but the members can be quite rude and scream scammer in a second. They do have a pretty good feedback system, but people won't tell the truth about a transaction for fear of retailiation feedback a lot.
pcenginefx.com 3 only used once and the seller was nice, but they really don't have much selling on the site. Their may be more PC-FX and PC-engines sales on this site but they are usually priced higher than other places. There isn't a good feed back system so its hard to guage sellers
http://www.amazon.com 3 its hard to get a good understanding of used condition.
http://www.ebay.com 1 and 5 This place can be great or the worst place. Very buyer friendly.
http://www.craigslist.org 2 Hit or miss really. I find that commincating with sellers is always slow and there is aways a safety aspect of meeting people.
A couple I would like to add
forums
Assemblergames.com 3 this place is the best to go for prototypes, betas, demos, and obscure games and systems. Other than that they don't have much and a poor feedback system.
Digitpress.com 4 a lot like racketboy, but a little more obscure stuff in the B/S/T forums. Their feedback system is kinda neat you get a thread dedicated to you, so its easy to read through.