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How can I limit the number of items a purchase from a seller?
You can set the number of items a buyer can purchase from a single seller either from MyeBay or in the AddItem request.
From My eBay:
Go to Account -> Site Preferences -> Buyer Requirements and select the appropriate values.
From the API:
Set the BuyerRequirementsDetails container.
Consider adding this requirement very carefully as it can potentially limit your sales.
Selecting Buyer Requirements