I also bought an older Thinkpad from eBay, and many others as well. Check their feedback, in particular make sure they have sold other laptops and not $5 socks.
Email previous winners.
Ask the seller for a phone number and call them. Then do a reverse number check. If they are not a business, get their work number. Make a bid and then check their details through eBay.
A good one is to say you live very close and would like to pick up. If they say no, be concerned.
Use a credit card if you can, or Paypal. Never use Western Union.
And always do a Pricegrabber/Shopping.com check before you buy. And check refurbs or clearance on the IBM site. I can't count the number of times I have seen people buy things on eBay for more than you can from a shop simply because they didn't check.
As for why they are cheaper, assuming they are legit, it's because they don't have overheads and they get the stock through non-official channels. Often the 'new' laptops aren't the very latest.
Given one of the best things about IBM is the three year warranty, I'd also check that the warranty is valid if you buy new from an eBay seller.
I bought my girlfriend a T23 which still had six months or left on the warranty froma private seller. If I wanted a real bargain, I'd be looking at a lightly used (2-3 months) one from a private seller. It'll still have 2 1/2 years left in the waranty, which is a lot more than most new lappies.
If you're concerned, post a link to the auction.