Even though their fees have become ridiculous EBAY is a good place to test market items before mass production. Disclaimer, if it is something that is easily copied EBAY is not the place to test market it.
Why?
There are significantly better channels for bringing a product to the marketplace. eBay fees are high, their customers are a PITA, their tools for sellers suck and you have zero opportunity to tell your product's story (i.e. why you made this, how you made it, why it's the coolest fucking thing in the category).
Shopify is the class leader here. Get some decent product shots, learn how to write decent copy and make a tight, 30 second video about the product (make it TIGHT - so many product videos blather on about crap). With those assets, you can plug them straight into a Shopify template with zero web experience and have a great looking store in a couple of hours. Bulletproof reliable, very low cost (roughly 3% of the sale, including credit card fees), great checkout experience for your customers.
After dicking around for years, we just threw out hands up and redid our website in a few days:
Luma Labs - Quality Camera Straps and that is 100% Shopify (we were doing a wonky thing with the front end before). Conversion rates are running at 3% since we did it (up from about 1.5%).
If you think you're going to move your product on a commodity website like eBay or Amazon, don't go into business. To compete these days, the little guys need to be premium, the quality needs to be there, the design needs to be there or the functionality needs to be killer. That's why startups mesh so well with machining, because machined stuff intrinsically brings a lot of premium qualities customers adore to the table, and you aren't spending $20k on molds.
Also, take your cost to make the widget, multiply it by 4x and that's your price. If your widget isn't sexier, more functional, better feeling or somehow better than anything else on the market, you are toast. People are totally happy to buy the nice thing for 2x-3x what the commodity on the market costs, but it has to be off-the charts better in **some** way (not necessarily every way).
(And if you think a 4x markup is high, my primary competitors are making their stuff in Taiwan for about $7, and charging $70. What they save in COGS, they spend on marketing and margin to stuff their product in as many channels as possible... it is a nice place to be, but since that spot is taken, we focus on different aspects/qualities and deliver a better value ratio, even at a higher price).
Edited to Add: Amazon is a solid sales channel to move to... eventually. Lots of Amazon users discover products there, but you need to have a solid brand built so when they Google before clicking "Buy It Now!" they see good things. You also need to move your inventory to Amazon, which can be tricky if you're hustling product out the door in the beginning - because Amazon's core customers Do Not Buy Without Prime. We are starting to experiment with Amazon now.