This question is referring to United States law.
If I get a breast pump from insurance and I don't use it, is it considered insurance fraud to sell it for cash?
It is not insurance fraud to sell medical equipment which was paid for by an insurance company. Unless it was obtained fraudulently of course.
Two warnings that probably do not apply to breast pumps:
Make sure that you actually own it. Sometimes equipment is leased; the patient does not own it.
Make sure that it is not a prescription item. You can't sell items which require a prescription.