Personally, I'm against 7200rpm drives in a laptop - I had vibration problems and loss of battery when I put one in my wife's white Macbook. However, times have changed, so this may not be as much of an issue any more...
Before you buy any drive, check its model number for performance:
http://www.harddrivebenchmark.net/
I recently bought a 250GB Samsung HM250HI to replace the pokey 160GB Toshiba in the Macbook - both were 5400rpm and 8MB cache, but the Samsung is a single-platter drive. It made a WORLD of difference - its even faster than the 7200rpm I have previously installed (a Seagate 7200.4).
Moral of the story: It ain't all about RPMs; a 5400 can be just as fast as a 7200 if its the right drive. Research first - buy second.
(BTW, OP are you dead-set on eBay/Amazon? If not, try
newegg.com - great prices, even better customer support. I can't recommend them highly enough.)