Well, if it's cheap then I'd say just buy it and don't waste his time (but if it's really cheap, then there is probably a reason...)
But, if it's pricey, then absolutely you should have him fill it, it's a pain, but if he wants to sell it, he'll probably do it for you.
I bought one tank off of craigslist (a 55g) I called the guy and told him I'd like to see it full, I set up a day to go look at it (that way we both weren't standing around watching water flow
) I told him if it held water, I'd take it, no price negotiation, and I'd told him I'd help him drain it too. He was fine with that.
Just my 2 cents worth