A marketplace by definition has to be a "market", e.g. have some form of prices and supply/demand. The currency doesn't have to be dollars -- it can be an imaginary videogame currency, for example -- but there has to be one.

So no, Reddit isn't a marketplace, any more than a library or editorial magazine or "letters to the editor" are marketplaces (which they aren't). Upvotes aren't currency -- you can't buy and sell them.

After a fashion. The "buying" party would be the readers, and the "selling" party would be... the rest of the Internet. The twist here is that some of the "sellers" don't know their content is being posted. But many do, which makes clickbait and marketing in general an example of the scammer problem that afflicts regular marketplaces.

Would say no as their is no economic transfer on Reddit... Yet anyway.

It might fit. It's the buying and selling influence. Mod accounts being bought and subs being taken over.

I've been told that it isn't a marketplace unless there's a financial transaction facilitated by it. However, I don't agree.