I was planning to make RAP gui above the player's head, however I don't have a clue to start this off. I just need some tips on how to get started, like using a certain service. I already know to use a billboardgui for this.
I'm not requesting people to make this, I just need some advice on how to start this.
I only need services or something to help me make this

8 comments
Using a BillboardGui.
That's obvious and I already know that...
You could look into APIs and Proxys which you can use via the HttpService to try and get sale information from the website (as you can't access the Roblox.com website directly from the game using HttpService)
Any clue where to look?
There doesn't seem to be a publicly-available RAP API, from the Googling I've done.
What is a RAP?
I've been told by this one Staff member to "send a singular request to a server that will accept your request. That server will then send the requests for you and do the calculations, then return it." But I have no clue how to do that.
I think the person you talked to explained how to utilize HttpService, which doesn't answer the question "What API do I use?".