How do you know if stuff for sale is real, fake, or stolen?

There’s a Facebook page for a couple of nearby suburbs; they call it the ‘Virtual Garage Sale’ or something like that.

Much of the merchandise is used (as I would expect), and at decent prices. Of course, there’s a lot of stuff I have no interest in, like baby clothes and cribs and football helmets.

There is a subculture of people doing commercial work (that is, selling new stuff that they may have imported or otherwise gotten ahold of). That bothers me since it’s a commercial endeavor, not a garage sale, so I ignore it.

Then there are people selling name brand – well known names – kitchen appliances, computers, TVs, and so on – sometimes still unopened and in the original box.

How do I know if the seller is legit? Or if they stole the item off a loading dock somewhere? Or put a used item back in a box and sealed it well?

Some of these name brand items have pretty good prices.

My question is: should I buy from these people, trusting that they got the merchandise legally? Or just avoid them?