Syncronisation Of Cs-Cart With Existing Eshops

I am often asked by potential vendors if they can somehow synchronize their existing eshops wiht their store in my marketplace.

I know this would depend on a number of issues, such as their current platform (so we are probably talking about custom development). What I am seeking here are some guidelines I can give (e.g, yes, its possible provided that x,y,z) and also, if there is any developer here who could give me an (approximate) quote I could pass on to me vendors for reference. If they wish to proceed, they can then contact the CS-CART developer to directly discuss on any details they need to.