I only use Coinbase. When you get the order total send it in BTC instead of cash. I’ve had no issues so far. Roidbazzar has the same kind of system. You need to send the exact amount for it to go through. There’s a button at the top where you can switch from usd to btcWith other sources it’s usually fine to do so and it’s a pain in the ass to get the exact amount with Coinbase due to a fee that’s charged. If I send $100 worth of Bitcoin, $100 does not get sent because a small amount is deducted from that total. It’s annoying as shit but I’ve always used Coinbase and it’s otherwise hassle free to buy and sell BTC. Have you used Coinbase or are you familiar with what I’m talking about?