We've done this in the London area and in Birmingham ... but have had totally zero luck up here.
We usually put up a Reverse Booking, specify exactly what we'd like to happen (as a minimum) and ask the bidders to comment (making this a requirement - ie. if someone bids without commenting, it means they've not read the RB and are blind-bidding).
The success we've had in London/Birmingham has been fantastic on occasion - so much so that we've gone on to make a couple of really good friends in each and even taken it as far as foursomes.
Boundaries -do- need to be set, of course... but if you're open minded, this can be a lot of fun.
It helps if you and your partner are both fit and good-looking, though, if you're looking to meet the same.