Sounds as though you have set to restrict number of purchases in 10 days.
See below, but unfortunately I cannot find where you can check your sellings
Updating your buyer requirements
You can set requirements to prevent buyers from bidding or buying on your items if they:
- Have a delivery address that's in one of your excluded postage locations
- Have a history of cancelled orders because they didn't pay
- Are currently winning, or have bought, a specified number of your items in the last 10 day