mysql - Questions on Auction Database Schema -

I was considering the following and I had some questions about it. I'm sure this is a good design because the man is appropriately qualified behind it, although some things do not understand:

1. Why did he separate the bidders and sellers? I thought that you will have users, and users can bid and sell items. In the context of the user table, you will have a bid table with reference to the user, and there will be an auction table. He is talking a lot in his tutorial to make sure that the models are scalable and ready for change (for example there is no status column, there are other tables and situations in context), so what is happening here ?
2. Why their plans are like "date of planned shutdown" and "winner." This data is not repetitive, because the last bid time (for the action that uses entries, using the last bid) The date can be calculated and the winner is the last bid when the auction is closed ..?

FYI: I am trying to make my own auction site scratch in PHP / MySQL and it is proving very difficult, so the tutorial will be great on this!

Thank you!

Why is it different from bidders and sellers?

The specific columns for each of the tables are unique, so they keep them separate. I actually go with the user and the sub-type bidder and the seller to the user, such as: / P>

  Tables User (UserID (PK), ... All General Fields for Any User) Table Bidders (UserID (PK, FK) ... All Fields for Bidders) Table Vendor ( User ID (PK , FK) ... specific field for all vendors)