Not sure a post count really matters but it's sometimes a helpful tool to determine how long an individual has been around. Notice that we lately have a lot of new folks?
Not sure I want to buy a car from a guy with 3 posts (not saying it's bad, but since people start to get "known" here, it seems like a safer bet to buy from a "known" member)
Also, while not 100% accurate, a higher post count might indicate that this "member" is still in good standing and has not been banned- at least not yet.
Personally, I preferred the feedback count (It's nice to know who you're doing business with and helps with accountability- it's one of the first things I look at on ebay- so, why not here?)... Heck, I'd put it right there with the post count. I could give a crap about "likes"... Just my .02
btw, I don't think your post count goes up if you post in the Marketplace sections either.