1. What does it mean?
네고 is a clipped borrowing from negotiation. In secondhand listings and shopping messages, it most often means asking whether the seller will lower the price. 네고 불가 means the listed price is not open to bargaining.
Also written or said as 네고하다, nego
2. The nuance
It is not an automatic discount. Broader business use may involve conditions as well as price, and a request should not be mistaken for an agreement.
3. Examples
“판매 글에 네고 가능이라고 적혀 있었어.”
The listing said the price was negotiable.
“네고 불가라서 표시된 가격으로 결정했어.”
The price was firm, so I made my decision based on the listed amount.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.