The suggestion is not that you return it but that you start a return case.

Then, if the part you are waiting for does not turn up, you won't be outside the period allowed by the Money Back Guarantee.

Hopefully, the replacement part is already on its way.

So, if you open a return and the part arrives, neither you nor the seller need to do anything further.  You can close the case or it will time out.  Tell the seller this is what you are doing.

However, if the part does not arrive, you can go back to the case and ask Ebay to step in.  Once the case in open, you have 21 days to ask for Ebay's help.

If you don't open a return case, and the part doesn't arrive, then Ebay will not help.