Neat idea, it's good to see innovation in the mining space. I'll have a rig or 2 pointed leaserig's way as soon as they are built! My only thought would be that an auction system could be useful, though I see that others have already suggested this.
I dedicated a lot of thoughts to this, but at the end concluded I will not do it. The reason is simple; price is NOT the only factor that should be considered. When you sell/buy a coin, price is all that matters. Here, other things are important such as: quality of service, stability, providers tendency to do refund or extra hours if something goes wrong, etc...
I will eventually reorganize first page - free rigs will be at the top and the ones that are going to expire soon will follow - sorting by availability. I will also add additional sorting options such as sorting by best price, and if once I implement rig ratings - sorting by ratings.