Buying a USED blackberry bold 9700 HELP? total newbie to BB's?

[Thingsssssssss]

I'm buying bell blackberry bold 9700 and getting it unlocked because I'm with the rogers network. Will it be use able? Also I'm buying this USED so I don't want to get scammed but I don't really know what to look for before purchasing. I've heard of the esn number? which frankly I don't know what it is .. how do i check this without having to call in ? PLUS who do I call in to check this BELL ( the phones original provider) ? or Rogers ( my service provider)? Is there anything else i would need to verify with rogers? Also I've been hearing about the bbm pin? or some sort of pin that is associated with the blackberrys.. Do I have to get that from the guy I'm buying from or do I ask rogers for a new one? The phone is from BELL so is the "pin" transferable to a rogers line?? I don't have a dataplan but getting the BOLD 9700 for the wifi since I'm at uni and wifi is endless. I jsut have a regular phone plan w/o data ..so if I suddenly decide to get a dataplan do I get the BBM pin new? Lastly , the wifi won't charge me right, how do I set it up so I'm using the wifi only and never actually using ROGERS network data so I dont get charged?
Is there anything else I should look for?? I'm totally new to this whole Blackberry world , haven't ever owned one so any help would be greatly appreciated . Also anything on buying used phones would be great too .. what should I look for?