Swappa is the most realistic, at least on the pricing side. They seem to, or used to, have really good controls in place for both buyers and sellers. Last time I poked around there though, it seems to have been overtaken by "phone reselling companies" and no longer individual sellers.
There are plenty of Pixel 3 XL phones on Swappa. Prices are kinda all over the place, likely based on the amount of storage capacity. If you go the Swappa route, make sure your carrier has fully "released" it from your account or Swappa won't let you list it. Carriers don't automatically do this either, even if you bought it outright, paid it off long ago, etc.
