I ask from the person who buy BTC. 10-20% average market rate. may be I don't know something.
The benefit for the buyer (aka scammer) is that he can use his phished or hacked bank accounts or credit cards to buy bitcoins which he can then safely and easily spend or convert to fiat. The 10 to 20% (or even more) price gap is immaterial and irrelevant for the scammer because the compromised bank accounts and credit cards linked to the PayPal account being used to pay for the coins aren't his in the first place so he doesn't really care about the "loss" by buying bitcoin at, say $10k per coin when the market price is just slightly below 7k.
Large price gaps should be seen as a red flag. They are meant to lure or entice people to sell their coins to these enterprising scammers.
