It would pay a person to buy a compression tester, and learn it's use. It could save you from spending your money on a troublesome motor possibly requiring a rebuild in the near future.
Here's a link, not the best info, but pulled it from a quick Google search, but it may give you an idea. If you can replace a sparkplug, you can run a compression test....
https://www.diynetwork.com/diy/ab_talk2diy/article/0,2021,DIY_13696_2556260,00.html
You could ask the current owner if he has a tester (since he's claiming compression is good....lol, which could mean it's good for him to sell it) and could show you the readings. Like anything else, don't take someone's for it, test it yourself or find someone who knows how & take them along.
ST