At first the title seems questionable, what is meant by server buying item?. Well server buying and sell items basically refers to a server GUI that buys items and sell items directly to the game (you instantly get the money). This GUI works like the auction house, however instead of being player sold, the game sells the items.

Example:
If you sell to the server 5000 wheat, you will gain like idk $100 you do you BUT this is a fixed amount taken by shop average so you cant name your own price, of course it would be lower than player pricing since it's pretty much wholesale and you get instant money.

The item you sell will be listed on the GUI, and the server will sell the item again as market price. When all the item (5000 wheat) is bought, wheat will be listed as "out of stock"

This feature should only buy agriculture items and ores with fixed price.
And like /ah it would only work in capital shop plots.

To sum it up the server buys it as wholesale you get instant money and server sells it again for higher price.