Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Doesn't really pay to buy used from what I've seen.... I'd rather spend $50 more and get it brand new
 
Search on eBay because there is a ton of people that are selling their used iPod's this time of year because of the new ones coming out next year. Also craigslist is a very good site to pick up deals on iPods.
 
Amazon is also worth checking out. Lots of people sell new and used iPods. I feel more secure on Amazon as well, the merchants have to follow Amazon's rules.
 
refurbished, period.
warranty, look like new. same for iMacs. I paid 994 for a 1499 iMac for my daughter, and just got the 8gb iPod touch for 149.00. true, might save a lot more on eBay and the like, but I hate flipping that coin and occasionally showing tails.
 
eBay
I bought a 16gb 2nd gen ipod touch brand new in Oct after I found out there was no new 3rd gen 16gb touch, My cost, 190 + free shipping, I sold it this month in Aug for 180 on Craigslist-before the new 4th gen iPod touch came out, So my total cost for a year of useage 10 dollars.

eBay is where its at.
 
Hands down apple store, they replace any defective components, put it in a new case, ran through the same testing as a brand new one to make sure it performs to standard, plus comes with the standard 1 year warranty.

Buying anywhere else..you get what you pay for. Sometimes amazon can have some good used iPod deals.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.