Advice for buying used? Fakes out there?

I’m wanting to start off with the Arc and gen 3 sub. I have a prepaid VISA card from ATT I was going to use towards the Arc at Costco if they still sell it and get the sub from Best Buy.

Then I got to thinking about looking at OfferUp. There are a couple of used Arcs for a little over $400. How can I tell if they are legit / latest model (is there only 1 Arc?)?

Do you guys recommend just buying “new” instead?