I am trying to find PHP developers for a series of projects of developing Wordpress themes that would go up on sale. These projects would be sold on a marketplace website and the payments handled in escrow but I am not sure where to find developers who might be interested in such a project? Does anyone have suggestions? Has any PHP developer done this before and would know where I can look to find?
What I am looking for is for a PHP developer to take a ready made HTML template and convert it into a Wordpress template but I am not sure where to find the developers. If someone can please give some suggestions where to find the developers.
Either you can find developer in freelancer.com or use me 
There are numerous freelancer websites out there dedicated to undercutting the market with cheap labor. Search Google if you want to start wading through the mess of freelancer sites out there. Freelancer.com is a very busy site and your project is pretty common stuff for that crowd.
IMO, go with a local web developer if you live in a city. You’ll be supporting local business and be able to have in-person meetings. If you don’t know any local web developers, get on LinkedIn and start building a network of people you do know - they’ll know someone locally who is a web developer. It won’t be as cheap as a freelancer site, but there are other benefits to professional networking that far outweigh the cost plus you get to know some new people you wouldn’t meet otherwise.
As others have said there are many many freelance websites out there. Google is your friend.
On another note, the PHP coding for WordPress isn’t particularly advanced. I only started learning PHP just over two weeks ago and already I’ve built a working premium theme, although that could also have something to do with me being a programmer in general.
What’s stopping you from learning PHP yourself and adding that to your existing HTML templates?
I have been trying to learn it but this is something I should be doing in more free time. Right now I just want to get these designs made into advanced Wordpress templates.