Here is how I would legitimize boost in this game. All trade chat boost is banned forever and all transactions are carried via Auction House only.
1. Only player who meets very specific criteria is allowed for boost ticket to be posted on the Auction House.
2. Buyer who is in need or interested in any kind of boost looks up available tickets on the Auction House and purchases it if they desire so.
3. Gold is not received instantly, but after final objective is completed special token is released that after that it can be redeemed for boosters gold.
This is just a fractional framework that can be expanded upon it in many different directions while keeping main idea in check. The main goal is to alleviate spam and scam fatigue as well as make this game look less degenerate by totally legitimizing whilst kicking out that big elephant out of the room.
It's a fair game for both parties.