Online auction sites like Amazon and eBay provide large marketplaces where specialty goods like comic books can be sold by shops, independent sellers or groups pooling goods into a lot. The algorithms that power those sites let potential buyers find Marvel comics and other goods easier than they might otherwise.
Amazon lists items for sale and makes them searchable by keyword or by name, author or other keys. This allows collectors of comic books to search not just for comic books in general, but for specific runs and titles within the larger genres and publishing houses. This makes Amazon an ideal place to sell Marvel comics.
While Amazon deals mainly in fixed-price item listings, eBay's stock in trade is auctions. This means that collectible items can potentially generate higher profits for their sellers by pitting various buyers against one another. It employs a system of listing similar to Amazon's in that it allows for easy searching by interested parties.
There are specialty websites like Comics Price Guide that instruct sellers in how to value and market their comics or collections of comics. With well-known titles like the ones run under the Marvel publishing name this can provide useful context and fiscal knowledge to enable competent sale and barter.