i would normally buy ram from a dealer who can offer a warranty, like newegg or crucial. but sometimes ebay is the best bet for obsolete stuff like the hard to find rambus i just got for a friend.
If you are looking for something hard to find, I'd scour the various Craiglist sites also. If you have a recent computer and are just looking for a good deal, I'd stick with the bargain RAM at Fry's/CompUSA/etc. I believe you have 15 days to test it out.
I've bought and sold memory on eBay. Just make sure the seller knows what he is selling. That is the biggest thing. Also, don't by it if they say they weren't able to check to see if it was working and are just selling it as is. The only problems I ran into when I sold some memory were questions from people asking if it would work with their computers. I would sell desktop memory and people would ask if it would work in their laptop and vice versa.
So basically, if the seller has sold memory before and can give you all the info you need on the memory (and say it is in working condition), then you should be okay.
I just sold a stick of Kingston HyperX DDR PC3200 on ebay for 96 dollars! You can buy that on Newegg for about $102. The stick was in great condition and never overclocked. My suggestion is to use Newegg, the shipping and handling are great on top of their prices. If you want it faster, order from directron.com and pick it up from their Houston store, they are equally competitive on many things especially memory. Though they don't have any DDR2 PC5400 yet.
I recently bought memory from crucial.com and I was very pleased with the price and their website. As I'm not a computer guru, their site told me exactly what memory is compatible with my particular Dell laptop model. They have pretty fast shipping as well. Good site.