[heinamj]

I just sold an item on the marketplace after exchanging private messages with the buyer and I want to leave feedback for him. I see where I can do it in his profile but it also says the preferred method is to leave it from within the marketplace posting, but there is no mention of "Feedback" anywhere there. I already marked it sold, should I have left Feedback first?

This is what it says in the profile Feedback area.
"Feedback may be left directly from this page, although we recommend you do so using the link provided in each Marketplace post."

[MarkJerling]

Have a look under your user name in your "Sold" post:

[Adam]

The easiest way would be to direct the buyer to the sold post so that they can leave feedback, and then just return the feedback from the link that is PMed to you.

If you want to leave feedback first, just do it from his profile and paste the link to the sold thread.