It’s a pretty simple solution to slow down botters without completely restricting the actual players.
Allow player a max of say 5-10 friends before level 150. They can unlock 5 more slots after a quest that provides other useful rewards making it something that’s worth doing. Then after so so much of the story quest or adventure journal is completed they can unlock a new slot every now and then.
They can also restrict the removal of friends within a certain time. This will clogg up the players and botters friend list restricting who they are able to trade with. Yes botter will just make more accounts and that’s something thats always going to happen. What it does however is slow them down and restricts their income to say 5 trades every month per account. Then if they want a previous account to trade again they have to delete their friends and wait another 15-20 days for it to be able to trade again. All this become very tedious for the botters but isn’t overly restrictive to players.
You could also either detect accounts that are deleting friends on a regular basis and ban the bot and the PLAYERS associated with them because banning the bots is irrelevant as they will just make another one, and I mean banning until a review of trades/chat/partying is done for the player. It’s pretty simple to detect interactions between players and RMT sellers.
Or you could simply restrict the number of friend removals per week/month again restricting bots efficiency to trade and not being an issue for actual players.