What can happen if i don't return the sellers item?

I have been waiting for months without not mobile phone 3 weeks now, i recently purchased a iphone 7 plus off a website called depop, using paypal as a payment method, now paypal has that the seller must not send the item until the buyers payment has cleared, my payment is in a echeque, this is because i transferred alot of money multiple times in my account, leading me to having nothing atm until the transfer is complete.So what happened was the seller already send the item, it should be here by tuesday, he has messaged me saying i should return it to her, but i don't want to because i really need a phone, paypal says that the seller must wait 18 for the echeque to clear etc. My question is if i do not return the package and use it, can he do anything or does he just have to wait? Because i really want to use this phone as soon as possible.

Added (1). Please excuse my english and grammer, as i was in a rush, thank you!

Added (2). Please excuse my english and grammer, as i was in a rush, thank you!

I do not understand - you sent an echeque that will not be covered, and you refuse to return the item.
That is called fraud and theft.

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