Making authors pay for access to admin area

I have a site that will sell works of art. Individual artists will sell their work and they will pay £X per year for that.

What I need to know is how would I go about the issue of getting them (the artists) to pay (via PayPal) before they can add their artwork to the site? There are ways I could do it manually (e.g. get them to pay the yearly subscription and then manually approve their username) but they are cumbersome and not very user friendly.

Can anyone recommend a method, or tutorial or a plugin that would help me out?

Thanks,

Kev