Do you send "thank you" messages? If so, why? If not, why not?
Not really, I don't find them necessary, unless I made the sale of my life.
Have you ever gotten a "thank you" message?
A couple of times, mostly in my profile.
How do you feel when you receive a "thank you" message?
Kinda like a mini-achievement, because I know how the other person feels, I've been in those shoes a lot of times.
Are there different kinds of "thank you" messages?
Hm.. It could go from a simple thank you to a chinawall text. Besides that, I don't know?
Do you prefer comments or PM if you had to receive them?
A PM, I find it more comfortable.
Why do you think people send these kinds of messages? Is there a specific, underlying, motivation?
It's simply to show a sign of affect, perhaps boosting his/her confidence as a merchant for further sales, thus, gaining a little more motivation.