Your marketplace sales history is not an asset you control. Ranking changes, fee rises and account suspensions all happen without warning, and every one of them cuts straight through your revenue. A list of buyers who chose to hear from you is the only part of the audience that travels with you.
There is a second reason. Marketplace buyers rarely remember who they bought from, because the platform is the brand in their mind. Email is what turns a one-off marketplace order into somebody who comes back to your own shop, where you keep the margin.
You do not need a big list for this to matter. A few thousand buyers who know your name is enough to launch a product without paying for reach.