How do sellers feel about letting a possible purchaser fly a glider before they buy it?

Personally I'd not allow it, certainly not with a high performance or complex glider.

I'd be open to refunding money if the buyer asked, provided no damage had been done.

I bought my current glider after my inspection, and looking over the logbooks, but didn't fly it before purchase. But it was a well-known good glider (ventus 2cxM). I did extend and start the engine and check the rpm, mag drops. I've been satisfied.