Upload & Sell: On
I usually try to get a sense of the seller's background, why they're selling, a story about who they bought the equipment from and if used then the story/motivations of that seller, their year of purchase, and I try to look up the seller a little bit to see if they're a real photographer. In this day and age, it is very rare to find photographers who do not have some sort of public web presence, at the very least on flickr or something. Legit sellers often volunteer some version of this info without even being prompted.
No one item is definitive, but all of the items together give you clues as to whether the seller is a real photographer, a seller of stolen goods, or a pure scammer.
You should also have your friend register the equipment on lenstag to see if anyone else has lenstagged those serial numbers.
I also try to avoid unboxed equipment or equipment missing stuff, unless I'm really confident the seller is legit. Some of the convos I have had on ebay with sellers have lead me to believe that some of that equipment is stolen: no boxes, no receipts, missing accessories, only the equipment that was probably in someone's car trunk. So unless its an older lens that has passed through multiple hands, I try to not patronize sellers of such equipment as the chances of it being stolen are higher.
If it's a paypal sale, paypal regular gives you the other party's full name and address. Paypal gift apparently doesn't. If some sort of corporate type name shows up instead of their personal name, that's a red flag (as I recently found out here on fm-- a low value item for which I didn't have my guard up that I got reimbursed by paypal for). If it's a cash in person sale, I like the above suggestion about asking them for a driver license and writing down the name, address and license number on a cash receipt.
I often tell people I'm trying to avoid stolen goods if they ask why I want this information. If they aren't more forthcoming with information after hearing that, you pretty much know it's stolen.
Apparently there's a pawn shop database of stolen goods. Maybe you can ask your local pawn shop if they'd run your serial numbers for you. If not, I don't see why law enforcement couldn't run them for you too. You could refuse to give the seller's info unless the item comes up as stolen, so you don't get someone in trouble unnecessarily over a hunch.