Well, the unit will power up using a standard USB cable, so you can verify if it works easy enough. Even if it doesn't for $150 you could send it in and get a refurb unit. So even though you had $225 in it, it's still a good deal. Now if it's stolen and the owner reported it to Garmin, then you're screwed. I sold a 550 a while back that was just the unit no cradles or anything because it came with a bike I bought. I already had a 550, so I kept the cradle that was on the bike. You can always ask the seller to give you the serial number so you can verify it's not reported stolen to Garmin.
Honestly, it sounds a bit fishy since the 550's really don't have chargers, they need to be plugged into a cradle because the battery life is so piss poor. Were talking like 2 or 3 hours, now the guy selling it really ought to know that shit. And if he doesn't have another 550 he's using there's no reason for him to hang onto the cradle since it won't work with anything else.