The Marketplace is somewhat buggy.
My last two purchases, I got a "purchase failed, will retry" message, with a note to contact support after 48 (or was it 72?) hours if things didn't resolve.
Both times, the purchase finally "took" in a day or two. I ordered the Aviat Husky and it just magically was in my downloads two days later. The other purchase (an airport) still hadn't shown up after the requested time, I emailed support, and right after emailing support (but before they got to my ticket), it suddenly showed up in my download manager.
So if you paid for something and got charged but it's not available in Content Manager, check again for the next couple of days, and if it still doesn't show up, file a support ticket at
https://flightsimulator.zendesk.com/hc/en-us
I'm generally only using the Marketplace if a plane is only available there, or if it's a plane/airport I want on the Xbox too. Otherwise, I try to buy from the developer's website, as that gives them all the money save credit card processing fees, or OrbX, since they have a good update manager. SimMarket is my third choice
(after vetting others' reactions to the product), and Marketplace is my option if it's exclusive or Xbox-friendly.
Dean has announced DC Designs will only be using the Marketplace in the future due to scummy pirates stealing his planes. Totally his call, as it's his business and I understand the frustration of seeing his hard work stolen. At the same time, I hope he continues to focus on civilian planes after the Concorde, because I don't want to pay for military planes that have had to have their gun barrels removed. (I've dealt with the ESRB since its inception in the 90s, and I would be shocked if they changed a sim's E rating just because it has non-firing gun barrels present on a historical recreation of a real aircraft -- I think this restriction is overkill.)