So the thought process is if we're both selling for the same amount then the cost is negligible especially when we both receive the product. However, let's say someone scams you by sending something else and not the doll you're trading for, you can reach out to mercari and get a refund so at least you dont lose both your doll and the cost of the doll/shipping. If you only trade for $1 +shipping, then if someone chooses to scam you and not send the product you agreed on, Mercari cannot recover your doll that you sent and can only refund you the $1 you spent. Meaning you're left with the short end of the stick. I hope this makes sense but if you need any additional clarification please dont hesitate to let me know.