Jay/fish40,
If you don't know how to code you can't do any of this. You won't be able to use any API or library. You need to either commit to a long learning process or hire someone to help. None of us have any knowledge of how mac4cash maps serials to devices. It's a core part of their business, so I'm guessing they won't share details, but you could always ask. My guess is a rules database they frequently update as new products ship, but who knows.
You have an ultimate goal, running a successful website that buys and sells used macs. One step you've established you think you need is a serial number lookup. Unfortunately there are a ton of steps in between. There will be a lot of complexity that hasn't reared its head yet.
You are asking us how to make a serial lookup. The fact is, none of us know. We don't need to do it. We can speculate, but no one will build it for fun for you. It may be worth visiting a site where you can contract developers, but the task you have has ambiguities and is a bit unbounded. This means the cost is unbounded. You have a tough row to hoe. Unfortunately we just can't answer this in a simple way.
Good luck.
-Lee