If you check out the bike in person, bring a couple of metric allen wrenches along with you. Starting from the tires that are on the bike, you can use the allen wrenches as feeler gauges to estimate how much more tire you could fit under the brake calipers and between the chainstays. For example, if the bike has 25mm tires on it, and you can fit an 6mm allen wrench between the rear tire and the chainstays, but only a 4mm between the tires and brake calipers, that's a sign that 28mm tires might work, but 32's are out.