How do I re-activate my model that i sold?

You need to have credits to reactivate your model, if you dont have credits, you need to buy credits.

I have credits, but I don't know how to use them to reactivate my model. Have you done it before? If so, could you tell me how?

As far as I know once you have enough credits it should automatically re activate.

Ones enough credits the automatic reactivation will work again.
You can upload more models to receive credits or buy the credits.

Discussion regarding this topic couple of day ago here (https://www.cgtrader.com/forum/topics/why-my-project-is-inactive#answer-2734)

I have enough credits, and some of my credits were taken away when the model was bought, but my model won't show up in my list of uploaded models.

It says I have bought the model to sell it again, but why won't it show up in my models list?

So CG credits going down when you sell a model is normal.
The model not showing up in your products overview is not normal (it should show up even when not active).

Had the same thing (model missing) couple of days ago, mailed to info@cgtrader.com about this and it was back again soon after.

They also replied currently having a bug that is being worked on.