That doesn't really answer the OP claim -- what are the realities of this?

I like the idea and can think of many rewards I could post, but the hard problem remains: how do we guarantee fairness? The reward poster would be incentivized to be picky with accept, the submitter would want to minimize the work to receive the reward.

One common solution to problems like these are reputation based (eg. eBay) or 3rd party mediation. There are probably others, but I don't immediately see anything done towards this.