...BTC opsec is not my technical expertise.
Would it be completely unreasonable/unsafe for a marketplace to run its own mining set up? Thereby giving all market transactions priority. If there is a way to do this, then I think the given market would gain a huge step above the others if only due to a reduction in BTC volatility.
To affect which transactions get included in a block, you have to be the one who mines that block. To reliably mine a block every 6 hours or so you would need insane amounts of hardware consuming insane amounts of electricity in giant warehouses and it would begin to consume your life and at that point the market would be an afterthought.