I would say so, they are not limited to a certain amount of shelf space or square footage like grocery stores are. On average, grocery stores work much more intimately with their vendors than amazon works with theirs (or did, now that they’re breaking into the grocery business).

I was mainly thinking of how an unlimited number of third parties can essentially sell whatever they please on Amazon, unless of course you’re google or apple selling a competing product that doesn’t help amazon achieve their goals for an ancillary service that has nothing to do with their online marketplace. I can’t think of a good analogy to a grocery store scenario.

Amazon calls itself "The Everything Store" and has enough market share that it is legally reasonable to think about if it is a deal facto monopoly governed by antitrust law

Monopoly? There is nothing stopping other companies from selling the same things Amazon does.

Any what is Amazon's share of all retail sales? My understanding it's in the single digit percentages.

Amazon had 4% of US retail sales in 2017, although that's 44% of US online retail sales. Online sales were 9% of retail sales.