You should also start seeing as many airplanes as you can of the type
you want. I didn't start to get a feel for prices and quality until I
had visited 4 or 5 airplanes and owners.

Now to your particular question, I'm no expert in C150 values, but for
damn sure a C150 with 2100 hours on the engine is worth $15K tops, and
as others have said, probably less. A C150 is not Sport Airplane
qualified so doesn't gain value there, and an engine rebuild from a
good shop with a good reputation is going to run, what, $12K-$15K?

As for the owner's reaction, that doesn't surprise me. Walk away from
that one and keep looking. And I'll add, you really should do some
more preparation. You state you initially thought $28K was a good
price...please...that's nowhere near correct.

--
I've finally learned what "upward compatible" means. It means we get
to keep all our old mistakes.
Dennie van Tassel