3rd party by definition means that there are 2 parties - the buy and the seller - and the 3rd party handles the transaction for the security of those parties. If you are both seller and 3rd party the buyer has all the risk.
3rd parties that a buyer can safely enlist usually have a couple of years’ perfect record, and have earned their right to be trusted. 1 month old character… okaaaaay
You advertise the sale, negotiate the sale, and take payment for the sale. You’re the seller in every way that counts. There is no 3rd party to hold the cash while the deal is done. Just you.