The scrap merchant is one of the 4 stations under the trader tab. Here you can destroy orbiting fleets to gain some of the resources back, but the amount is greatly reduced, usually around 35% of the original ship costs. This amount can be increased by "negotiating" (paying dark matter).
If you don't have dark matter, then the scrap merchant is probably not a good option. It would return about the same resources to use the ships for a moonshot, especially on high debris field universes. Also, you'd get a chance for a moon. Of course, this requires a large fleet with recyclers.
If, though, you happen to lack a large fleet, recyclers, or a buddy to moonshot trade with, this may be a more reliable and less risky option. Also, in case you built a huge amount of solar satellites on a planet you placed in slot 1 to get graviton technology and after achieving that goal you no longer need the planet and decide to abandon it, you can make good use of the scrap merchant to recycle a part of the resources spent on the sats.