So I can trade these six strapping African lads for 45,000 board feet of lumber? How about this young Thai lady, she's barely been used. There have to be limits on the acceptable vehicles of trade or Humanity is guaranteed to do really fucked up things.
So if we accept some reasonable limits you can trade whatever you like if you can find someone willing to recognize the value of what you're offering. You can trade oddly shaped pebbles if you want and ascribe any value you see fit. If there are a bunch of pebble traders you all might come to an agreement on a standardized value of the pebbles. You and your buddies are free to trade pebbles all you want.
But I don't want your pebbles, I don't care what their equivalent cash value is. I want cash, so it's on you to convert them to cash, not me. You want my thing, I want your cash, not your pebbles, your pebbles have no value to me.
By using a unit of trade that's not recognized by the person you're trading with you've effectively reversed one of the few near universal constants: You've put the seller in control of the transaction. Normally the buyer is actually in control of a transaction as your unit of trade can be applied to other things the buyer wants so the buyer actually has the need, you as a customer fulfill that need. But if your unit of trade isn't widely accepted then it's equivalent value is greatly reduced.
It is perfectly acceptable for me to ascribe a much lower value to your trade unit as it isn't fully functional. So an item that cost $500 in cash would cost you 10 Bitcoins. Since there is no widely recognized value of your trade unit I can value it however I like and you can either deal with it, convert it to cash, or try to find somebody who recognizes the value of your trade unit and is willing to deal with the hassle.
No legal recognition means your trade unit has the value I set, not you. If you bring cash that value is set by a third party so there's no room for gouging, it's an even playing field. So why not just deal in cash? Why set yourself up to pay extra for something?