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Sure, you have a dashboard to manage your inventory levels. I've been shipping a shit ton of product and haven't gotten anywhere close to limits. They also will spread your inventory across multiple warehouses

How much of your inventory do you sell and how fast do they sell on average?

It's all subjective. Retail products move way faster. Books will sit longer, unless you find ones that have faster sell rates. All of this data, you can get from something like Keepa. It will tell you the sales raes and such for items