They certainly can increase your sales, theres quite a few now – most of which you mentioned.
eBay, Amazon, MyDeal, Catch, Kogan are the main ones but as per usual with marketplaces, you really need to read their terms to understand their rules, fees and their processes.
You can definitely grow sales and add another sales channel but they all take their cut from your sales so you have to weigh up whether it is worth it or not.