There is a difference in size between the tramps for an adventure Island and the tandem island. So just make sure you know which model these tramps are from. But any tramps from any year tandem island will fit any other year TI.
If your not getting a big break on the price for the tramps, then it might be better to buy new from the dealer. This way you get the two year warranty, and also the 2013 and up tramps have much better and stronger buckles (another un-documented Hobie ongoing improvement).
I think the earliest tramps that came out in 2010 had steel rods vs the newer fiberglass rods on later models (the fiberglass ones are better)
Other than that they are all the same, and should last for years.
PS if you ever break a buckle, if you take it to the dealer, they will replace broken buckles on the spot for you. In the 3 1/2 yrs I've had my TI's with tramps, I have only broken 1 buckle, the tramps are a very nice option to have, especially with kids (lots of kids-LOL).
Hope this helps