Don't discount Ebay; yeah, pricing can suck, as well as shipping, but every now and then there's a great deal. For example, I found Mega Man X3 and 7 both for 40$ because they had label wear; I just printed out new labels. I also got Mega Man 5 and 6 cheap just by "make an offer".
First, spelling and punctuation are your friends. Second, I don't trust Ebay; there's too many folks willing to rip folks off at every turn.
I can't seem to find these "folks." Out of 51 transactions to date, I've only had one "bad" one, and they just lied about the condition. All it took was 5 minutes and a paper towel to remedy that one

. If you know what to look for (recent seller history, etc.) then there's very little chance of getting screwed, and even if you do you have both ebay buyer protection and paypal protection. So there's literally no way to get bamboozled on ebay unless you're lazy or don't read into the seller and listed info.
I've actually had much worse luck buying locally like Wiggy. Craigslist has been okay, but all of the local stores around me have things in poor condition with sky high prices, and they openly sell bootleg games and CDs for their Japanese imports. To top that off, you have no buyer protection buying locally - often they won't let you return something, especially if it's broken (regardless of whether you received it that way or not). I've also been openly laughed at about some game purchases at a local store to boot (by the cashier of all people...), so they're not getting my business anymore. There's a few good ones around despite that, but like I said, I almost have to do more research to buy locally than online...
Do you mean your friends as in make sure to spell correctly, or the way I'm taking it which is to say that many listings contain improper spelling and labeling?
I believe it was the "Condescending Grammar Nazi" kind of way.

For every shitty transaction on eBay that I've had, there are probably about 20 good ones. I'll take that ratio. I've actually had worse luck on Craigslist, and in those instances I get to see the products first-hand before the purchase. Still, I've had many CL items end up being totally bunk when I get around to actually using them versus watching a guy plug something and turn it on in front of me to let me know that it "works".
Man, we have TONS of retro gaming stores in Cleveland. I guess I should be thankful, but it means that you don't find that much at garage sales and whatnot because it's easier for people to just take their old games to said stores and trade them in for $$$. Then I'm stuck paying retail for SNES games most of the time :/
This pretty much identical to my experience. eBay is an awesome tool for finding something specific and getting it at a fair price. I wish we had better stores around here (Boulder/Denver), but there's really only one independent store that's worth going to. I think finding vintage video games at garage sales is getting less and less common as they age and gain value. I find lots of xbox games, but not so many NES or SNES games. Wii and Xbox 360 games are practically free, but there's so many filler titles that the games worth buying are few and far between. eBay just seems to be the way to go in the current game market.