... first, I'd say don't. If you can get it for $450, go that route -- buying a used i500 is just very risky!

But, if like me, you like saving money and have time to correct problems that may arise, you may find buying a used i500 an attractive option. If so:
[list=1][*]Make sure the phone comes with everything you'll need. A lot of the sellers on eBay only offer the phone. A cradle and extra batteries will cost extra money plus time, shipping, and their own buyer's risks.[*]Get the seller to post a picture of the phone's system history (dial ##RUN on the number pad and hit "OK"). You want to know how old the phone is and maybe whether it's reconditioned or not.[*]Make sure both the screen and Graffiti pad are fully functional and unscratched! These are the most vulnerable parts of the phone, and many buyers adopt a "don't ask, don't tell" policy about the condition of these critical elements.[*]Do all the regular homework you'd normally do, e.g. make sure the seller has a good reputation, etc. I personally prefer individual sellers on eBay for bigger-ticket items, though if you're not careful they are hard to distinguish from scammers.[*]Get a no-DOA promise from the seller or better return policy (for things like unacceptable screen condition, etc.).[/list=1]
If, like me, you end up with problems on your phone, hope is not lost. Obviously you can try negotiating with the seller, but if that fails, or if you want to resolve things on your own, head to your nearest Sprint store and ask to have the problem fixed. There's a good chance at any given store that the person there will simply decide to order you a new (reconditioned) phone. To do this well you'll have to "forum shop" -- I lucked out and found a sympathetic person on my second try. (The first person said that "Scratches aren't covered; try a screen protector on the Graffiti pad").

If you don't get software as part of your package, you should download and install them in this order:[list=1][*]Samsung USB drivers[*]Palm Desktop 4.1 (not the most recent version)[*]Chapura PocketMirror if desired[/list=1]
Links to all of these should be in the FAQs to this forum.

Anyway, hope this helps someone... I spent a lot of time on this phone, and probably paid way too much, but now that I have it, I'm glad I bought it! It's certainly better than my god-awful VisorPhone!