It's a difficult one.
I'm still trying to get my post count up on another site from my old car just to put an add up. I've been a member on there for over a year but not much or a poster, more of a reader.
I got round this by posting in a non-for sale section, the mods removed the content but it was still clear from the title what I was selling.
I am now dealing with people based on PMs but not ideal as it did not open me up to negotiations with the wider audience.
Might be a hassle but made any newbies selling should have their transactions mediated through the moderators as like a middle man.
Personally, selling the stuff from my old car I am looking for people localish to me so deals can be transacted in person.
It's a hard call.
On the other hand, being new myself I'm not likely to have much for sale (unless anyone wants my ralliart carbon wing mirrors) but will most likely be sniffing around for the odd bargain in the for sale section!
50 posts is achieveable in a realtively short time if you do have a genuine interest but agree it can lead to people just posting for the sake of it!