I need someone to advise me on my ER diagram.

I learnt about ER models sometime ago and now I need to do it for one of my sch proj (but cannot recall much)...

I need to design a webportal with a few functionalities in it..

This is one of the specification:
Auction
-Members can view either by type, or all available items at once. Sort by type,brand, bid number (i.e. “HOT”), top bid, start date (i.e. “NEW”), and closing date
-Members can post items for bidding
-Members can upload the picture files of the items for bidding
-Members can bid for available items
-Members can remove items from auction (only for owners of the item or
administrators)
-Create “Category Type” (top-level) and “Category” (second-level)
-Remove “Category Type” and “Category”
-Remove current auction item
-Access log of Past and Currently available items
-Clear log of Archived items
-Access log of Past and Current bids
-Clear log of Archived bids

I also need to include shopping cart, but my problem is: I'm not very sure how to link the shopping cart entity with an entity named transaction(for payment and delivery of products user purchase).

These are what I wrote:
shopping_cart
-purchase_id**
-quantity
-cart_id

transaction
-transaction_id
-cardholderName
-creditcardNo
-creditcardType
-expiryDate
-purchase_id
-deliveryAddress
-deliveryDate
-purchaseDate**

(**I don't where to put this attributes.)

auction_item
-category
-sub-category
-itemName
-startDate
-closingDate
-topBid
-startBid

Question is:
1)Are the attributes I identify appropriate?
2)How shld I link shopping_cart & transaction, so that the transaction entity knows what products the user adds into its cart? Using the cart_id?
3)Where shld be my category or sub-category be? Shld they be another entity? because there might be a lot of category and sub-category. Or shld they be just an attribute?

Sorry for the long question..
Hope someone can solve my problem..

Thx Thx..