"Why are toilet seats non-returnable?"
For hygiene reasons, we cannot allow or accept return of any toilet seat or bidet seat once it has left our warehouse. When you order your seat from us, you can be absolutely certain the seat is brand new, and has never been used by anyone before you. Once a seat has been delivered and has been in the possession of the purchaser, there is no way for us to be completely sure the seat was never installed or tested; even if we're told the box had only been opened to look at the color or design of the seat, how would we then be able to guarantee the cleanliness of that seat to any other customer? We want you to rest assured any toilet seat you purchase from us is sanitary, brand new, and was never used or installed by anyone before you. If we accepted the return of toilet seats, then we could not make the same guarantee to the next customer.

"When I received my toilet seat, the manufacturer's box was not sealed and had already been opened. Did you send me a seat that was returned to you by a previous customer?"
The seat you received was brand new and unused by anyone before you. When we receive a shipment of seats from our supplier into our warehouse, their boxes are often opened for inspection, to make sure the seats are not damaged or defective; we also inspect the seats before we ship them to you, to make sure they're correct and in good condition.