sid
username
consist of
1...1
Books
Foreign key:username2
Item
User1
0....1
User
firstnamelastnameaddress
Foreign key:sid
Haves
Foreign key:username
user2
itid
Wants
Foreign key:hid
titleauthordescriptionvalue
Exists
username1username2daterate1rate2
ISBNcondition
hid
In
Foreign key:username1
Swap