Sold a never used Ryobi generator, buyer says it doesn't work

Bought a Ryobi generator for $700 from Home Depot just in case of emergencies. I took it out of the box and it has been in my garage. I never used it and decided I was going to sell it on Offer Up. Get a bunch of offers and finally someone commits to buying it. They pay me in cash. I get a message later that night saying it doesn't work. I look up a bunch of videos because it has never been touched so I am skeptical about it not starting. I sent a couple youtube videos I found on getting it to work and tricks to start it up. They said they would try that out. Took a day to respond back so I assumed everything was all good. I marked it as sold and moved on. They sent me a message on the 3rd day having it that they still couldn't get it to work. How would you handle it? I am a little skeptical of buying it back because I have no idea what they have been doing with it while it has been in their possession, been about 6 days now. But I also don't want to scam them if it for real doesn't work, but also don't want to get it back if it is something they did and now I am out.