Look at the edges of the deck and the under side really good. Make sure the baffles aren't bent up or damaged. Look at the top of the deck where the pulleys are. If they've hit things with the spindles and did serious damage, the top of the deck around the pulleys will be bent, the paint won't match the rest because they've heated it up with a torch to bend it back into place, then repainted it, etc. Check for abnormal whinning in the hydo pumps, take the drive tires off and look at the wheel motors, pull the dipstick out to look at the oil color after you ask them how long it's been since the last oil change, etc.