App Store Review Guidelines 3.1.1 - Allow users to convert their virtual currency to USD?
I don't see anything in the guidelines that would prevent you from making such an app. However, it is always best to check with Apple as they are the ones making the final decision.

There's no need for you to actually implement this as "buying back gold". You can just let users buy an in-app purchase that is "gold", that they can use to tip the content providers.

Then on your web site you can make a section where content providers can login to see how many times they have been tipped. This doesn't need to have anything to do with "gold" or in-app purchases or anything like that.

As you write that the withdrawal (payout) is made through your own web site, and not the app, this sounds perfectly doable.

As for legal issues, you need to contact a lawyer in your area. Laws are different in various countries, states, etc. and could depend on how you have incorporated, and other details. In particular you would probably want to ensure that everything is setup correctly in terms of taxing.