"Etsy" plans as download - license, restrictions?

It's up to the seller to specify the license the buyer acquires the model. Very common in those licenses is "no resale of the model" as well as "no sale of derivative works of the model." Derivate means by the was both altered as well as acessory to an item. Some have also express terms for selling manufactures of the model.

By downloading the files the buyer then is bound to the licensing agreement and has to abide to them or be liable for willful copyright infringement. Mentioning the source you had only shields you if you have express permission via license from the owner - but claiming you would have a license when you have not might put you into extra hot water for misrepresentation!

By reuploading the files you break that no resale part of the license, in addition to possibly breaching your contact with Etsy as established in their ToS.

Etsy has a terms of service. It's pretty clear you would break those rules by selling the same thing someone else is on a site for homemade goods.

As for the models you build, obviously the same rules that apply to any models apply to yours. You can sell them or keep them or do whatever you would like.