Hello Tony!
1 - if the models price is higher the activation fee is higher too. The more expensive your model is the more royalties you get. We want to have highest quality models so that is why activation fee for cheap models leaves you with almost half of the price :) Check here https://www.cgtrader.com/pages/sell-3d-models
2 - Yes after the sale the model is not seen in our library. But if someone is looking for a model on Google it will show up and the buyer will be able to buy it. and you would need to buy credits for that sale too.
When you have inactive model you can not delete it and if you do not buy any credits for a long time the next payment goes to our account and we give you credits. It is just a fee to keep this marketplace running.
Also if you buy more credits the model will be activated automatically so you do not need to activate it after every sale.
If you have any more questions feel free to ask :)