Yeah, I think my current mindset on the matter is a mix between most of you guys on here and ClassicPhilatelist. True, it's possible the stamp itself wasn't genuine, which makes me feel better. Also, for someone who claimed he wasn't interested in stamps to do all the digging, research, find a reputable dealer, etc., it's probably not likely. For that reason, I feel better on the matter. I also appreciate the support from you all in telling me not to worry about it. Plenty of fish in the sea, or stamps in the postal service so to speak.
Then again, there were the other 299 glassines. Granted, I saw a lot that were in fairly high scott numbers, but some mixed in with ones that clearly weren't, such as a group of 3 cent jacksons from the 1800's. And the glassines all had multiple stamps, so it was pretty silly for me to take that risk. One of the more bizarre glassines I could see in the box were two glassines of 2 cent Columbus stamps, with 100 stamps in each glassine. I haven't been collecting long, but I see no reason for someone to have that many 2 cent Columbus stamps unless he was a dealer, a careful philatelist, or a weirdo. Makes you wonder what kind of stamps in there might be genuine: maybe some more 1800's stamps of moderate value.
I don't have many hard feelings for the guy: I mean, he did find and buy the box at an estate sale, and he did do some digging upon my request. Then again, he did have the opportunity to simply leave a voicemail about how he wasn't selling the stamps, and instead left one saying to call him back just so he could brag about his finding on the phone and hang up. On that note, I hope they're all fake

Regardless, I still slept pretty well, haha.