Going cheap often means getting hosed with the PIN being reported stolen or registered with another carrier. Suddenly "cheap" doesn't look like such a bargain after all.
You can look at the Buy/Sell/Trade forum here and trade only with members with a good amount of feedback. Looking at their post count can help you weed out new members who might not have much history here. Likewise, Crackberry.com has a lively marketplace. Same rules apply: Check history of seller and feedback.
Personally, I'd rather avoid the hassles of buying from unknown traders unless it's someone like a forum moderator or long-time member. My first choice would be to spend a little more but get new/unlocked from Horizon Wireless
. It's like getting it from RIM directly. Class outfit, great customer service, and guaranteed to be legit. (note: their prices are all in Canadian Dollars)