Unless there is paperwork to prove WHAT work was done 'rebuilt' could mean he gave it a tune up.
Master parts kits are $800 or so, machine work is another $500. You never said what the asking price was, but that would also play a role in how much of a rebuild actually occurred.
Even running it for 30 seconds won't tell you much.
Scrap is $10/100lbs. A 235 is about 600lbs so scrap price is $60... and a full rebuilt motor is $1200-$1500 if DIY.
Now your part is how much do you want to gamble on this guy's story? Me... unless the motor is still wrapped up from the shop, rebuilds do not put much value into an engine... how many miles? Why is he selling? What the motor look like now? All those questions take $100's off the asking price.
And no matter what a seller says, scrap won't be more than $100
