Can seller lock/erase my second-hand Mac with Find My Mac?

I can confirm (from second-hand bad experience & first-hand testing) that a complete disk erase & clean reinstall does not disable Find My Mac. You need to also either:

• Reset the Mac's PRAM (aka NVRAM aka firmware settings). You can do this by holding Command-Option-P-R at startup, and holding them down until the computer has restarted & chimed three times (actually, it's reset by the second chime, but three is traditional).

• Set up FMM with your iCloud account. Since the Mac can only be FFM'ed with one iCloud account, this'll replace any previous setup.

I would do a full wipe on any new computer I bought anyway, regardless of iCloud.

Regardless of that -

Before buying it, I would want to see it turned on and working. Then have the seller go into iCloud preferences and turn off everything, then log out of iCloud on the machine.

Basically, have them follow this: http://support.apple.com/kb/HT201065?viewlocale=en_US&locale=en_US

Bear in mind that this works both ways - if it's still on iCloud you could potentially get their messages, emails, photos etc, so it's in their interest to take iCloud off it fully.