Older Sky kit you can sell on if you want to but,as far as I know, all Sky Q kit actually belongs to Sky and has to be returned to them at the end of your contract.
You could use them as additional Wi-Fi Access Points (AP's,) additional ethernet switches, or a combination of the two, though so doing will entail establishing a cabled link between them.
The "Using Two Routers Together" FAQ pinned in this forum detail how.
Most ISPs (and I assume Sky are the same) will supply postage paid bags on request to return routers, TV boxes etc. Probably better than them ending up in landfill.
Yes, they do, here's the link : Sky: Return or Recycle Equipment