Great idea, O benevolent One.
The availability of a pre-buy test would certainly remove a lot of the uncertainty, for both buyers and sellers, since it seems much of the stuff on hamfest tables is untested. Even the small bux stuff can be powered up and tested in a few minutes. (I recently got an 'estate fresh' Drake R2B going just after the guy bought it, by wiggling the tubes.) And if an obviously cooked component or dead tube is discovered, what better place to shop for replacement parts than right at the Fester.
As for $$, I think the "free" aspect is truer to the spirit of ham radio. Also, tinkering, troubleshooting and fixing radios apparently has a certain entertainment value, as shown by the number of people far more expert than I who have stopped by the table and end up spending serious time helping out.
Coming to Marlboro? I have a spare white jacket.