This is (slowly) being discussed among mods.
LDS is (sort of) correct, SMF does not successfully limit minimum post counts even though there are options for it. (They are currently enabled but SMF doesn't seem to be doing anything about it)
This has also been discussed in the past, in that some people don't see a problem with "random joe" coming in to sell parts they no longer need.
I also don't see a problem with this, as it's just another source for people to get cheap(er) used parts.
So long as the post satisfies the other rules of the marketplace, I don't think there's a problem with sell and ditch.
This is of course subject to buyer beware. Same as any other interwebs sales, there can be some great deals. On the flip side, someone with no post history on WSC is just like buying from kijiji. You don't know the person.
I suspect that the "5 post min" rule will be removed shortly.
But a note to people with 0 post counts or those who have just signed up with the sole purpose to sell, that is no excuse for not following the marketplace post requirements. (detailed information, pics, contact method, shipping)