I think retail can be improved, and even small improvements can have large impacts because of the sheer size of the market: US retail sales are in the $5 trillion range yearly, mostly in brick and mortar.

Specific problems:

1. Why is brick and mortar still so popular, and can any pain points with e-commerce be fixed?

2. E-commerce doesn't work well on cheap items where shipping cost is prohibitive. Different companies have tried to solve this in various ways, with Prime (losing money on cheaper sales in hopes they can reduce logistics cost and drive larger sales) or Jet (directly giving shipping savings for ordering multiple items at once). It will be difficult to compete with Prime, but there has to be an angle that works, Jet found one.

3. Simpler price comparison. I tried to build the feature I thought should exist at https://icanpriceit.com/ as a side project, but didn't spend the time to properly launch it. I hope some startup succeeds in that space, I've been watching https://wikibuy.com/ which is quite similar.

I think there's plenty of room to build the next Amazon or eBay. The fees they charge third party sellers have been going up over time, if a marketplace was willing to accept lower fees at first it could help early growth.