You can take a compression reading, but the problem is the oil gets all over the rings & can fill small scratches in the cylinder walls You could have 150+ PSI on the gauge after clearing out most of the oil, but then after its up, running & really cleared out of all the extra oil it could actually drop as low as 120. You just never know. By all means check it to make sure theirs nothing really wrong, but don't assume 150 PSI means its all good either. Not trying to be pesimistic, but it could happen.
With 4-stroke engines that have low compression, it was common for guys to dump some oil in the cylinders to see if the compression would go up. If it did, the rings were failing, if it didn't it was more than likely the valves. old school trick.
If its a really nice ski otherwise, it might be worth nabbing. But your the one taking the risk, so the price has got to be to your advantage. Oil in the case, yep the crank seals are leaking. Might need a crank, but might just be 3-4 years of weeping oil thats no big deal at all. Compression checks good, might be a strong top end, but it might be lower than 130 & masked by the oil.
I guess I would go into it it knowing those things could be wrong. If they're not, its a bonus! If there bad, your not overly dissaponted. most guys won't pay more than $200-$300 for a non running ski of that vintage, but if its really nice otherwise you could up it a little more IMO.
BTW: I doubt there's a buyer at $800, or he wouldn't be talking to you about it. That's probably his way of telling you what he wants. Half that price could be "iffy"