When you edit your game, halfway down the page there's a button labeled "Upload game". It lets you upload from your computer, no need for Dropbox.
To make your game for sale, choose "Paid" higher up, under "Pricing", and enter the price in the box below.
Once you're done, look at the bottom for an option called "Public". Choose that and save.
But! If your game is for sale, an admin will have to review it first. It takes a day and a half or so, and you can see when it's been approved from the referrals in Analytics. In the mean time you can post the link on Twitter and such.
Hope this helps!