If you plan to travel Seoul,  please Be careful about Food Stalls at the Gwang Jang Market. 

1. Please check the price and record it. For example, the menu says 8,000 won when you order, but, the seller will say "It is 10000 won".   Cases found that seller add some other food without asking, and they charge it.

2. Please ask and check if they take card (credit or debit). According to Korea law, it is illegal to refuse the card for purchase.  However, some sellers at the Gwangjang market refuse the card for purchase and it is for tax evasion.

3. Please check their sanitary conditions.  It is reported there are some non registered sellers. 

4. Generally people report very bad Customer Service. 

It is better just to skip here instead you may want to check out other street markets where local Koreans go.