The more I hear about this idea the more I don't like it.
When I want something I what to know exactly what it costs, not have to play a guessing game.
A system like this only rips off the consumer.
Example:
Item usually sells for 5k.
You post it for 3k.
I bid usual price of 5k.
You steal 2k from me.
No thank you.

Personally I prefer honesty in the market, they way it is.
And not a back alley shady deal they way a hidden market is.