Typically, Manufacturers warranties are not transferable. I don't know about Tamron but I believe that to be the case with them as well. When buying Any used item, unless it is coming from a dealer with some sort of secondary warranty (Mack, Square Trade, etc) you should probably figure on it being As Is. That said, with any lens of this type, I would be looking at past history with it. That is, is it well known for having problems (like the DA*16-50)?
Buying used lenses is a great way to go and fill gaps in the want/need list IMO. In making your decision though, weigh the potential (Again, Potential) loss from paying for a repair against the cost of a new one with your Own 6 year warranty (in this case). If the seller is only offering a relatively few dollars off of the cost of a new one, buy the new one.