The model perk was controversial between Rubik and I during development. The original plan was to give supporters all of the models out right, private or not but then the Lifetime rank was born.
You have to understand that the people who are life time are probably veterans of the server and are purely here to support it (hence the name) or they are people who really just want to play the server without the cosmetic grind and instead want to play for x y z reasons. Also don't think every player is going to have the rank. If too many people buy lifetime, the price will be increased as I want the rank to be rare.
People who have supporter do not own your model and they cannot put it on their class. This means I cannot go on a player's class with supporter and use your model because the supporter only has access and doesn't own it.
Your best bet is to either put competitive prices for models or make them private and advert them in the discord or in game. As long as the model is private, players cannot use them. Players with supporter + lifetime can still buy your model if they don't want to always use !ms to use them, you'd just have to put competitive prices for that.
I may make it so if you own custom content you can create a "token" and sell it on the auction house. At least that way your automated selling might not be hindered as much.