Man, that's the classic used motor gamble right there. You save the customer a few hundred bucks up front and then eat a whole extra job for free. I swear half the used motors out there are just time bombs waiting for a new sucker to install them. Last time I got burned, it was a "low mileage" Honda engine with a spun bearing they'd hidden with thick oil. Now I try to at least pull the valve cover and pan if the seller lets me, but even that's no guarantee.