I get about 5-15 MSN invites and at least a dozen PMs a day. A lot of this stuff helps me communicate with other trading parties, or otherwise simplify the trading process.
"Add me - [email protected]" messages are extremely helpful to me, as I know which person to initiate a conversation with, and I can find their name easily using the search option in MSN.
"Added" is also somewhat helpful because I know that I can now expect to talk with them on MSN. Again, it allows me to use the search option in my MSN window
"I'm interested in Acc 3 --
[email protected]" is also useful. It tells the person to expect a MSN contact about the sale, and it also lets them "search" for my msn and type a quick, "Hey bro, still interested?" Also, it can preliminarily prove that my MSN isn't hacked.
"PM Me" helps tell other people to PM with the same question; it also helps me because typing "PM me" is a lot easier than actually PMing the person

There's a 30 second cap on messages, but there is a 60 second cap on PMs. They can overlap, and I often post in between the 60 seconds per PM response.
"Talking to you now" is
extremely important. I use it to link my MSN email with my Sythe account, basically to prove existence of my Sythe account. Why not do this with a PM? You'd never believe the idiots I've seen. I've found it can save me at least 2-3 minutes per trade just asking them to post rather than PM. ("Durr, what's a PM? What's your username? How do I PM you?") I figure they've already seen my thread, so asking for a post there (and responding with one) is just much, much, much, much simpler. Plus, it's there forever now--I can use it in a scam report, whereas having to dig back 5-6 days worth of PMs is fairly arduous for me and a lot of users on here.
As for "free bumping", "nice account", and off-topic remarks are spam and have always been considered spam. Mods don't have thousands of eyes, so it's our job to report the posts and the users will get warned/infracted/temped according to the severity. It's the mods' job to judge the punishment, not ours.
One last thing: these are all very marginal methods of "boosting" post count. The market isn't the only forum that adds post count. For every time I post "Talking now" or "MMing this now", I've posted ten more times in another section like Runescape 2. I'm one of the apparent "few" who think post count is a bunch of garbage anyway. I looked at Gohan's 14k for the first time like last week, and I was surprised he had so much. I'd honestly never seen his post count before that.