I think what they are trying to say that is if you are an "older" member that has been here for a while, you should have some feedback score to use as a gauge. If the members are new, chances are that there is no feedback for them and you are taking a bigger chance dealing with them than an established member with some feedback/history to go off of...
Some people just use the site to join and sell their stuff. some of them trying to ask extra high prices as this site has their target market and they try to inflate the prices.
But that isn't a for sure thing. There is a thread going on right now on a guy who ripped someone off that signed up around 2010....
You need to ask the right questions, and feel out the seller to see if you feel comfortable dealing with him, estabilshed with feedback or not. If not, move on to the next one for sale, even if it will take a while - better later than ripped off...