I have had scammers reply to WTB ads several times.
They all have claimed they had what I was looking for, usually they send you a picture.

I have found a pretty simple way to help weed out weather the person actually has it or not. I ask them to take (whatever they claim they have I was looking) place it on a scrap piece of paper and then on that scrap piece of paper, hand write my email address on it (or phone number if the exchange is via phone).
That is usually the last you hear from them if they are a scammer.

But there seems to be a common one going around lately. When I ask for the above picture, they will send you a picture of a person's drivers license and say something to the effect, see, I not a scammer, here is my info, go ahead and send the money, (or ask for your info like name and address). I reiterate I will wait for the picture before I send any money or info. They lose interest and stop after that.

That has happened to me at least 3 times. Each time I never sent any money or personal info to them. I was not going to until I got picture proof they had it (which they can not produce).
The last one was just last week. They guy was asking a lower than normal price for a barrel I was looking for. That was my first small red flag. However his back and forth email skills seemed like a normal person. The barrel they had was not 100% the exact model I was looking for and I mentioned it. They came back with another picture saying they had one of those as well, but it was clear it was not the correct length and was for a older model gun. That was my 2nd red flag. I then asked for the picture with my email address on a piece of paper and then got the reply "it is at work and I am not there right now, what address is this going to?" and then they say they are not a scammer, they are a law enforcement officer, and then send a picture of a badge with the persons drivers license with it. That was my last red flag. I know then they were a scammer that was trying to get money and personal info.
I do not know where these people are getting these pictures. I have thought about trying to contact the police department that was on the badge and see if I can contact the real person and tell them someone is impersonating them.

However I have had a couple people respond to my ads and they had no problem taking the picture I asked for, and then the sale went through like normal.

You just have to be smart about it and be real careful about giving out personal info.
I use gmail, and usually it will display your real name with your email address. I changed the setting it to where it does not display my real name, and instead just displays "Jeeps" as my name. That way, when scammers email you, all they get is your email address and not your real name. Also my email address does not have my name in it. (it is very similar to my screen name here). So no personal info there either.