Let's have a look at what you've done so far, then we can take it from there.

I agree with Liam, your database needs to show data once and once only, so rather than having a field of 'Dealer/Seller' (for example) in your PCComp table, you have a separate table for dealer, with all the details such as address, telephone etc.This table has a primary key, DealerID, which uniquely references that dealer, and this key is used in you PCComp table.

(sorry i didn't explain that too well, I've got a cold!)