GO

GO = Group Order

The seller will ask if anyone is interested in also purchasing an item they are going to order. If so they may launch a 'Group Order' so they can buy the item for you (and others interested) in one order, and you buy it off them when they receive it. This often reduces costs for the buyers. It is typically used in large buy/sell spaces for items like K-pop and Anime merchandise.

Seller: Is anyone interested in purchasing this album?
Multiple Buyers: I would like to order some but shipping is too expensive.
Seller: Then I'll open a GO and order them for you, and you can buy them from me for a discounted price.