Move the bars smoothly side to side and look for any signs of notchiness or binding that may indicate dented or even cracked steering head bearings. Especially around center.
Grab the front brakes and PUSH the front end down and let it up. Try to feel through the bars for any signs of odd behaviour. It should be smooth with no notchiness of any sort or sounds other than some squishy sounds from the oil moving around.
Get down and look for signs of fork oil around the seals. They should be clean and show no noticable film of oil.
If it passes all these checks, and the rest besides, then the guy knew how to land them decently softly. If you find it has one or more of the problems from above then factor in some money to fix it.
Oh, and it's a long shot since the bikes are pretty good about this but look carefully around the steering head welds and a few inches back down the frame for any signs of cracks. Use a rag to clean away any dirt and bring along a small and sharp knife to check for any signs of what looks like a crack. If the tip of the blade applied lightly just cleans the dirt away then no problem. If the tip goes into a crack and doesn't want to slide sideways easily then you say "thanks, but no thanks" and walk.
But if all thes checks fail to find any fault then you're good to go. The fork seal life MAY be reduced but it won't amount to a hill of beans. It may turn out that the steering head bearings will have a reduced life span but if you're happy that you're getting a good deal on the price then suck it up. Replacement bearings shouldn't be more than $50 to $60 plus labour to install. If you need them have it done while other work is happening and the labour will be lower.