Post the types of GTA services you could provide and get them exposed to thousands of players in one go. You can provide GTA levelling/rank boosting service to increase in their accounts rank, max their GTA Skills/Stats, drop money and items for their characters, or any custom services that you are able to do.
PlayerAuctions will facilitate the transaction process to ensure the security. All you need to do is register yourself on PlayerAuctions and post the details of what you are powerlevelling for your customer. We will make sure that you get the money once you deliver what you described.
In your product description, remember to make it clear what you are offering or power levelling for the buyer. Try to use the Grand Theft Auto V jargon where appropriate to make it more understandable to buyers what you are providing. These include how much you are going increase their rank and stats by, what items will you obtain for them, or how much money will be in their account when they retrieve it. And also importantly, how long it will take to complete all those services.