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.