So, you’re confused about something, and you post on the forum. Someone answers your question. Perhaps you need to ask for a clarification, perhaps you don’t.
Either way, if you have something to tell them, do it on the forum, within the same topic.
Don’t send them a private message on the forum, unless they invite you to.
Don’t send them a message on the main site. The only reason to message a seller on the main site is if you intend to buy from them, and need to ask them something about their service.
Don’t spam. Just don’t. It’s not nice, and if that’s not a good enough reason, here’s another one: you will probably get reported, and your messaging could get disabled.
So, don’t do it.