There are a lot of scammers in the P2P market, and that's why, whether you're the buyer or the seller, you have to confirm that everything goes well for the other party first.
But the burden is with the seller when most of the buyers are scammers, and they'll fake it that they've paid them already.
So, always verify when someone is very active in the P2P market because the scammers aren't choosing their victims. As long as there is a potential victim to them, they'd scam them.
You pointed out big problem with P2P market which is that there are many people trying to trick others. It is good for both buyer and seller to be careful and make sure other person has done which he/she must do.
You also mentioned that it is mostly harder for sellers because some buyers will pretend they have paid when they have not. This means sellers need to double check that they have received money before they send anything.
It is also good idea to be careful of people who are very active in P2P market. Scammers mostly try to do many deals so they have more chances to trick someone. So if we are using P2P trading we needs to stay alert and check everything carefully to avoid getting scammed.
Yes, it's the responsibility of the sellers to check every transaction that they go through. Because if they're careless and just doing it based on the trust they're giving to the buyers, they're mostly going to get scammed again for a period of time.
That is the reason why we have to verify each transaction that is paid to us. This gives everyone more caution because it didn't happened just a few times but for so many times.
Those that are very active in the P2P markets, all you have to do is to verify every single time you have a transaction in that market.