There are sooo many "SF used" things on eBay that make no sense. I can totally believe that sometimes there's a logic I just don't understand, sometimes the guy is just weird... but more often than not, someone throws stuff together, slaps "SF used" and BAM +50% value (or so he hopes).
It looks like an actual Ceradyne gunfighter cut but with fake Ops-Core Maritime rails, straps and Headloc. Probably fake Wilcox too though I can't really say (maybe someone better than me can) with that paint. My guess is that someone threw it together to make it "look cool" and hopes he's going to sell it for $750 rather than the 300$ it may be worth if sold separately.