Rich people problems
It depends on the contract.if the contract was 'selling the weapon with the design files' then you no longer have the right to do so. otherwise you free to do what you want.
If it was a tradeable weapon that you sold, then you don't own the model anymore. If you only sold the model of it, and the weapon itself is still in your inventory, then you still own the model. This rule doesn't apply if you acquired permission to use someone else's current weapon's model.Read the guide
That applies to existing weapons in the game, not files. If you sell existing weapon and you don't have files then you don't own that model anymore. If you only sell model, and weapon is still in your inventory, then you can keep selling model.If you have files and originally own them, or you bought ownership for files, then you can keep selling files