Selling Online

Having a bit of a clear out. I have a few things I could probably sell as opposed to taking to the dump as well as some stuff I want to put on for free. The free stuff are CD racks, a TV stand and a glass metal shelving rack. They are all in one piece given away so no issues with any comeback. The stuff I want to sell is a Tacx bike trainer and an old home cinema amp, speakers and speaker cable. They haven't been used in years, the only testing I've done is plug them in and the power lights came on. I have NextDoor and Gumtree, if I sell these on there is there come back if something ends up not working? Are they sold as seen or should I put that on the ad? Just to know what I'm letting myself in for over the head of £100 odd quid.