There are several excellent resource publications available, which provide regular updates on the values of coins, antiques, and other collectibles. Many may be found at your local library branch, or even requested at the reference desk.
While they may not always give you that very moment's market value, these magazines and newspapers can help determine if an offer is legit or not. It's also helpful if you can't get to a pawn shop or similar dealer, or if you're just starting out in collecting rare items.
"Numismatist" is a good starting point for coins; "Wizard" for comic books, figurines, and replicas; I've seen several for stamps and antiques, many of which are long-time standards.
Unfortunately, the scam artists are known to operate on aliases, burners, and suitcases, so the best thing to do is like the others mentioned: let OfferUp's Support staff know, file a report with the authorities, and just let karma handle the rest.
I did not actually, but just went through the police and they basically just said live and learn, And I didn't check the PS4 box till after because she had her neighbor distracting me and my buddy and he kept saying what's up what's up, and it was in bad projects and as I pulled up she changed her pic to an all black picture, So if any seller has those all black or all red pictures do not buy from them, And if I would have opened the ps4 box I believe I would of got robbed or shot. It's best to just meet in a public place with surveillance