If there's a skin that I want and I wear the demo how can I export that skin, remove the demo logo and use the skin itself?

You can remove the demo wrting on any texture by using photoshops content aware fill procedure (Photoshop 2020 is posted here somewhere) , but for a skin it will be hard to get it perfect.
Best bet would be to camp out at the store and wait till you see someone wearing the skin and copy it. However, the currents Hydrastorm viewer you can only copy a baked skin .i.e. It will have the hairbase , make up and tattoos the person is wearing, baked onto the skin you have copied.