The problem with Amazon is you won't find one unlocked...
Though If you save that much money on the phone you should be able to send it off to be unlocked.
(Remember that deal is only for new T-Mobile customers signing a 2 year contract)
There are some threads on this forum from people who've had their 8100 unlocked for something around $99 and said the guy was very reliable...
Also you should be OK buying on E-bay. It's easy to have a bad experience on E-bay if you buy from poor sellers.
This auction is from a guy with 100% positive feedback, almost 150 received. I can almost guarantee you a pleasant transaction if you buy from him. eBay: NIB BlackBerry Pearl White Unlocked 8100 (item 110087976477 end time Feb-10-07 17:25:43 PST)
(given his feedback rating; I buy and sell a lot on e-bay and having that many positive feedback, keeping it at 100% is not easy and is a sure sign of a good ebayer)
So in other words just look into the seller alot before you buy