WordPress as reverse bidding site

Hi … I am planning to develope a website using wordpress, similar to elance/freeancer etc. where clients can post projects and interested people can bid on those projects. Was looking for the best wordpress theme that can do this and accept paypal payment. The idea is that when client pays to the person who has done the job a percentage of that amount would be transferred to our (website owners) account. The client should be able to rate and leave a comment about the person who did the job only once the payment has been made.

Any help on how this can be done would be appreciated.