I sell on Vinted but nothing expensive as people only want to pay pennies. For example, I am selling a few things for my Auntie. I recently put on a nice top for £1.50 which was definitely a good price, maybe to cheap. I received an offer for £1 so I ignored it. The following day the same person bought it anyway for the full £1.50! Usually, they won't buy it if they don't get their discount!
Other people don't quibble over the price as they know it's a good deal.
The only things I have purchased were a couple of nice summer shirts for DH as the ones on sale in the shops this summer were dire. No problems with them at all.
I use InPost and Royal Mail only as these are the best options for me locally. The buyer pays the postage.
You can have bad buyers. My neighbour sold a very nice skirt through Vinted. It was perfect, never worn labels still attached. The buyer wouldn't pay because it was damaged. It was clear by the photo of the ruined skirt, she was several sizes too large for the garment. Unfortunately, Vinted upheld the complaint despite being sent the original photos as evidence of the condition it was when sent. My neighbour didn't even get the skirt back. She won't use Vinted again.