Yes, you could buy stolen bitcoins from an exchange or elsewhere, but no, you're not going to lose them. Nobody can take your bitcoins away from you; the worst they can do is refuse to accept stolen or otherwise "tainted" bitcoins as payment, which is hideously impractical for a variety of reasons.
This keeps being said, but is misleading as it confuses how Bitcoin works.
No one can know what kind of coins are going to be sent to an address before they're sent. And no one can reject or refuse to receive bitcoins sent to their address once the address is given.
Should a merchant/exchange receive bitcoins that they trace back to a crime and don't want to deal with, they
may return them. Or they might keep them (as "evidence" or whatever.) "Refusal" would really be doing either of those, and it's the latter that's the problem; if you're going to be bold enough to hold onto someone's money yet openly refuse them the service/product they paid you for, you should be bold enough to involve law enforcement or arbitration to get to the root of the problem as well.
So to answer the OP; yes, if you send money to someone who is tracing the bitcoins, you might "lose" them if they trace back to a crime and the merchant/exchange is unscrupulous and keeps your money without involving the police. Such tracing isn't a widespread practice among merchants at the moment, but just as with U.S. dollars, if it happens that they decide to keep your money due to a claim that the funds are stolen, you should consider immediately involving police or arbitration to resolve the situation.
