The warranty part, i am not sure on.. hopefuly someone like moewmix can come around and answer that part.
As for the stolen part.. your BEST bet, before you fully pay for it, with the other person there (or if they are willing it send it to you before hand) is contact rogers, they can check the imei number.. if it was reported stolen, etc.. that number should be blocked.
When you buy a used Rogers phone, call Rogers and have them add the IMEI number into your account or whoever account it will be under. The warranty ONLY works if you are with Rogers. Blackberry does not cover anything unless Rogers sends it to them. That is the only way warranty will work. Also make sure the person selling the phone, releases it from their account. That would help more. The IMEI number must be associated with a Rogers account for the warranty to work.
When you talk to the person, ask them for the IMEI. Once they give u the IMEI number, Call Rogers and see if it's been reported stolen or anything bad. If Rogers says it's fine, you are set. Also i would suggest doing this deal at a Rogers store. That way the store rep can see if it's been stolen or not.