Don't pay anything upfront, not even the shipping cost, when buying used items online.

A trader was offering a used product for sale. I contacted him and he asked for a deposit. I told him I could only pay the shipping fee in advance. He agreed, took my address, and was supposed to ship the product. The next day, he called me and said I had to pay a 25% deposit of the product price because the shop owner told him he must take a deposit. He had already shipped the item but hadn't confirmed the shipping company yet, and I had to send a deposit to confirm with them. I told him to cancel the order and send me the shipping account. He said he couldn't because he had already sent the item to the company. Obviously, this was nonsense and I got scammed; he was also greedy for more. There are no chat screenshots because he deleted them somehow, or maybe this happens after blocking.