I'm not sure this coinmarket cap link helps the original question much, as many / most of those Exchanges listed aren't available to US citizens ... maybe Bibox?
Doubt that any exchange will help. Most of the ones I tried consider Bytom an ERC20 token, however mined Bytom is going to be on the mainnet.
I think this is going to be an issue that most people purchasing the B3 aren't considering. $400/day sounds great but it's all funny money until you can either use it or exchange it.
Full disclosure I preordered this knowing that this could be an issue. I took a gamble and hope that by May 10th other exchanges start listing Bytom.
Kucoin seems to have a reasonable volume, shouldn't have any trouble there as a US citizen.