Why do you presume that crypto marketplaces will have zero or less fees than traditional markets? I am sure crypto related ecommerce websites will have more fees when coupled with transaction fees which will also eat into your profit margin.
Decentralized model is not always sustainable when it comes to an online marketplace that sells physical goods. Also the trust factor matters a lot! Don't stay under the impression that everything decentralized is good!
Because with a decentralized system it does not have to be profit oriented. You can just set the thing running and charge only what is needed to make it work. Ebay have a lot of staff and other overheads. Not that a decentralized system would always be great. If you need support you could be out of luck depending on how it's operated.
Darkweb is the biggest marketplace of internet and you could buy anything you want by paying cryptos. Also use market place section of bitcointalk as temporary solution in case if you need it.
Like I said I don't want to use the darkweb / tor.