I'd assume LAMP skills. If you know your way around multipool, I'm guessing you could probably put this together.
Some knowledge about working with payment APIs, too. I'm not too sure what language you need as I'm not a programmer.
LAMP = Linux Apache MySQL PHP
So the language is PHP
If I wasn't a programmer I'd be scared to death of putting a technical project together because you won't know whether they know their stuff or not. There are so many bullshitters out there. Try posting on freelancer.com and they will all come out of the woodwork.
Hope you've got a programmer you can consult with to see if the people you employ have genuine skills or "made up web skills". There are way more of the latter doing the rounds on the freelance websites.