if I sell something on marketplace and the buyer wishes to pay m with paypal how does this work ?

Answer 1

Chances are you are dealing with a scammer.

Selling stuff on FB Marketplace or Craigslist should only be done in cash an in person.  While PayPal is legitimate, scammers have perfected fake PayPal emails that make it look like you are getting money when you are not.  If the person is asking you to ship the item, it is definitely a scam.  If the person is claiming to be out of town, it is definitely a scam.

Answer 2

Don't do it.   Get cash or postal money order.  

Paypal payments can be reversed. 

If it's a large transaction and you live far apart, you could use an escrow service.   The buyer sends the money to the escrow service.   Once it's received, you send the thing to the buyer.   Once the buyer receives the thing, the escrow releases the money to you.   If there is a dispute, the escrow service is there to arbitrate.