That's not really true. OTC markets still need to acquire the Bitcoin they sell to their customers. If they don't have enough on hand they need to get it on the open market just like everyone else.

It would be a bit like if you had 1 BTC and your uncle asked if he could buy 2 BTC from you. You could technically sell him the first one at any price you like (although, you're still incentivized to charge the market price) and the second one you'd have to go and buy on the open market.