I've been to PAX West the last 8 years and went to PAX East once in 2016 (I think, maybe it was 2015). Expect a lot of craziness, the Boston Convention Center is huge and very crowded, but also organized. Seattle is much more spread out but also harder to find stuff. If you go for AAA games expect long lines, if you go for Indie stuff, still long lines, but less long usually. If you don't care for Swag, you won't have to wait in line as much. PAX is so much fun, but it takes some time to learn how to get the most out of a convention and it can get frustrating to get shut out of stuff you want to do. It's best to just try to go with the flow, plan ahead for stuff you MUST do/attend and be open to new things.
There are tons of awesome giveaways everyday, it's often about being in the right time and at the right place, and there are also plenty of places to just buy swag. I recommend following the PAX subreddit and checking in on the tagged Swag thread- people share where they got cool shirts, posters, games, etc...
PAX East did have some retro games sellers, but most were hilariously overpriced. I did actually snag a copy of Vandal Hearts II and Gekioh Shooting King for a good price after some haggling.
All in all, PAX rules and I hope you have a great time.