All of that stuff is good to check after you but it, but not practical before hand. You can check the cylinder through the spark plug hole.
I don't see any seller letting you take their saw apart to do a vac check. That's just a risk you take when buying a used saw. Besides, it's cheap to fix.
Compression is usually checked on a cold saw.
About all you can do is a visual inspection and see if it runs good, maybe a compression test. IMHO, asking to do much more is intrusive to the seller.
Brad, I would agree with you if he is paying less than $300.
But if someone is paying more than $500 or $600 for a 10 year old saw. I would highly recommend having it checked it out. And if the buyer is not able to check it out, then find a dealer and spend a few bucks to have it checked out. I'm guessing but a dealer shouldn't charge more then $40-45.
For example, I'm currently talking with someone about purchasing their 3 year "new" Stihl MS460 (with 10 hours of use??) for around $650. There is NO WAY I'm going to buy this saw for that kind of money without checking the piston for scoring, removing the muffler is 4 bolts. And if your taking the spark plug out to look at the cylinder wall, then may as well do a compression test. If the seller objects then I will walk away.
Helpful note:
I haven't proved this yet but I believe a very simple seal (vac/pres) test is to warm up the saw and sit it flat and see how it idles. While the saw is still idling place the nose down for a few seconds and observe the idle. It should be the same. Repeat this process nose up, clutch side down, and clutch side up. I believe any change in idle represents a leak in the up area.
Anyhow, I'm just trying to be helpful. I don't want to see anyone pay $600+ for a used saw and have to put another $200 into it. Good luck with the purchase.