There is no money to be made on refurbishing (what an abused term) ThinkPads unless you have such a source. Buying them on eBay and attemtping to re-sell at a decent profit margin is next to impossible. Take it from someone who's been re-selling ThinkPads for a decade.
George is correct on this.
I do refurbish systems, but very, very few of them come from eBay. Occasionally I find the odd mis-listed system which I can get a good deal on, but that's exceptionally rare.
Most of my source machines come from Craigslist; between the RTP providing tons of ex-corporate machines and the universities providing plenty of computer-illiterate, recently-graduated sellers there are quite a few deals to be had here. Even then I wouldn't recommend it unless you also enjoy tinkering with hardware. (I do, and I tend to use my refurb sales to allow me to change my laptops out regularly. It's definitely not a major profit source...)
Need help with Linux or FreeBSD? Catch me on IRC: I'm ThinkRob on FreeNode and EFnet.
Current laptop: none
Current workstation: IBM Intellistation 9228 (running FreeBSD 9.1) - blackbird