Is the bidder a seller or a buyer? Or is it possible for both? If so, under what circumstances is the seller? Under what circumstances is the buyer?

The buyer is the tenderer and the seller is the bidder.

Bidding is to reduce the cost of procurement or engineering. So it is the act of buying a house.

A unit is a tenderer in one environment and a bidder in another.

For example, an engineering company participates in the bidding of a project construction, and his identity is both a seller and a bidder. After winning the bid, in the construction of the project, it is necessary to purchase equipment and materials, so he becomes a tenderer and leaves the country as a buyer.

Personal opinion, for reference only.