Actually, nickjb has a point, and one assumes that you have checked but what does the proof of delivery show on the website of your chosen courier? Have you verified that the package returned is in exactly the same condition as you sent it and contains what you sent and not a counterfeit swapsy… Also did you address it correctly and send to the address registered on the eBay account?
Check those things first and if you make contact with buyer do it via eBay so it’s all recorded in their system. Often buyers contact details are included when they purchase so don’t be tempted to ring them up/send a text. If all genuine then proceed as conscience dictates.
Or, are you asking as you have buyers remorse and want your money back hence “asking for a friend “ …? If the latter then I would just contact the buyer and ask . If it were me I would offer a return and refund when I got the item back and had checked its condition..