Do you plan on offering on multiple platforms? Amazon? Etsy? Your own .com retail? A separate .com wholesale?
Which other sites you want to use can affect your choice for your main platform. You want your main platform to have full integration with the axillary sites so you can see your inventory, orders, customer interaction being pulled into your platform and also the platform pushing new products, inventory levels, tracking numbers, etc out to the other sites.
Squarespace is pretty good on the social media integrations, not so great on the nuts and bolts successful product based business integrations so you would probably want to use a hub platform like shipstation.
Some of the names I see a lot for ecommerce sites outside of the large marketplaces are:
Shopify
Big Commerce
Woo Commerce
3d Cart