1.
Users: {{UserID}, country, location, bidder-rating, seller-rating}
Items: {{ItemID}, UserID, name, first bid, buy price, number of bids, started, ends, description}
ItemCategory: {ItemID, Category}
Bids: {UserID, ItemID, Time, Amount}

{} indicates a key

2. Aside from those that include keys, there are no dependencies between tables.

3.Yes
4.Yes