Is the fact that something is sold "used" mean a buyer can always return it, if it doesn't function fully, even if the seller claims they said it was untested in the ad. (but also claimed it was 'used' and not 'for parts of not working'?)
No, not necessarily. Regardless of what ebay/Paypal policies say, they don't trump your country's consumer laws.
For example, in Australia, anything bought at an auction (including online auctions) comes with no consumer guarantees. This means even if the item is broken, faulty or not as described, according to the law the seller has no recourse to either return the item to the seller or seek a refund. However ebay or Paypal may offer a remedy to the buyer, but that's up to them.
However making a "purchase" is covered by consumer laws and in such cases, the seller must offer a remedy (and can't fob you off to the manufacturer). They are also responsible for reasonable costs incurred when shipping the faulty item.
Even international sellers who are selling a product in Australia are bound by the Australian Consumer Law, however enforcing consumer laws can be difficult or almost impossible when it comes to international sellers.
At the end of the day, do your homework, study the photos carefully, ask the appropriate questions (never assume anything) and not only read what the seller has written carefully but consider what the seller
hasn't written. Deliberate omissions can be a sign that the seller knows there is something wrong but is failing to be honest about it.
Also do some research and understand the consumer laws in your country and what rights you're entitled to. If your consumer law rights have been breached, the seller must offer a remedy and if they refuse to, ebay should be taking appropriate action.