That's the nature of FB Marketplace. Way too many lowballers. I just sold a iPhone 7 Plus 128 GB. Over two dozen inquiries before it finally sold. $240. I mean, I can sympathize with some people going through tough times, but...come on.
Thankfully, I also bought a new iPhone Xs Max 512 GB. No haggle. $650.
It all depends on the buyer, lol.
i was gonna post on other sites like redflags or craiglist to get a better feel of the market for 11 pros. 256gbs