Are you in the "Facebook Ukulele Marketplace," or just looking at Ukes listed in the Facebook Marketplace? The former is a group that you have to ask to belong to; and the other are just listing in the general marketplace. The group usually has better listings and sellers.
Facebook reaches a lot more people in a lot more geographic areas. Some people have no background in ukuleles and have very unrealistic expectations about what their ukes are worth. Others are looking for a fast sale. Unfortunately, there are also more scammers, or sellers that aren't as forthcoming about problems or damage tot he instrument they are selling.
I've found that it's best to ask a lot of questions about the instrument about things that aren't mentioned in the listing. What strings are on it? Is the fretboard radiused? Strap buttons on it? What kind of wood is the binding, etc.
Really look at the details in photos. If they are low rez, ask for high rez to look at. If there is an area of the uke that isn't shown in the photos, ask to see photos of them. Like neck joints tot he body, or the butt of the uke. I've had a couple of unhappy occurrences where the seller was very careful to not show parts of a uke that was supposed to be "like new" and turned out to be damaged. Also, beware of glare and reflections that can be hiding dings, dents and scratches. Usually it is not intentional, sometimes it is.
Obvious red flags are when you find the same identical photos of the uke under a listing from another seller, or the photos are taken directly from the mfg. website but are supposed to be the actual uke they are selling.
Check if you can call them to talk about the uke on the phone. Scammers usually don't want to do that.
The biggest thing, and it sounds like you are already doing it, is tif possible o find examples of the same uke and what they are selling for new and used. Then go from there. Always be willing to walk away.
Meet in public places. Some police departments actually have a place in their parking lots specifically for online deals.
There have been some other threads about scammers on the Forum and how to avoid them. Pay attention to people that use Craig's List. THey usually have good info about meeting up with sellers or buys and how to spot scammers.
Good luck with your searches.