Actually, there is plenty of opportunity in every niche. eBay is just a very tough market, and as gets posted here at least once a day (it seems) prices on eBay are generally below wholesale.
You are not going to find much in the way of dropshippers and distributors who can offer you prices that will compete with eBay and offer you any profit worth having.
The opportunities are boundless; it's the approach that is limited. People buy into these dropshippers and similar outfits with expectations built on sales copy, rather than market research. Then when reality sets in -- sour grapes about the market.
You just need a different approach.
->Pallet and truck quantity orders from Liquidation, TDW and other reputable places
->Buy from within your local market and sell on eBay ... many many people do that, buy at stores having liquidations or going out of business then resell on eBay
->Buy on eBay and resell to your local market. Even in this day and age many people prefer to handle something and see it in person rather than take a chance on having to hassle with mailed returns.
->Buy on eBay and sell on eBay. Lots of wholesalers on eBay (lots of crap
wholesalers too, so be careful!)
->Go with eBay's affiliate program rather than trying to compete head-to-head. For that matter go affiliate on everything rather than direct sales; it's a lot cheaper up front and can be very rewarding.
->Add value and bundle; combine different products + a tutorial or something like that (would vary widely since 'product' can be almost anything) and compete on innovation rather than price
->Compete on service/warranty/etc. rather than price
->Deal in secondhand, especially books, luxury clothes/accessories and tech that you can refurbish. Many newbies weirdly turn up their noses at secondhand when it's a goldmine. Used books alone ... what a market.
etc. etc. etc.
Opportunity is abundant!