My advice for checking your transom rot would be to remove a few
screws/bolts if you've got lots of stuff bolted to the back.

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Another good method with stringer, transoms, and other places where
there is concealed wood is to drill a small diameter hole and look at
the wood waste scavaged by the bore. It it is bright and "crisp", the
wood (in that spot) is probably pretty good. Dark and soft, or worse
yet, damp, and there is a problem.

One problem: It can be difficult to convince the seller to allow you to
drill holes in the boat. :-)

If everything checks out, the holes can be very easily filled and
sealed.