Do most stores in Seoul accept credit card payments?
If so, what types? VISA, Mastercard, American Express?
I went to Seoul in summer 2015 and I could use my Visa everywhere. I cannot tell if there are some places which will reject your mastercard, most likely there will be but I've stayed there for a month and I could pay and withdraw money without any issue.
As far as I know, Koreans have plenty of credit cards and they use it on daily basis so you should not have trouble using yours. ;)
I did not have my Mastercard so I could not try but most ATM were compatible with it. I didn't check for American Express since I don't own an AE card.
EDIT2: as said in one of the link, don't forget to notify your bank that you'll use your card abroad... For security matter my bank was denying any operation from abroad, I had to tell them where I was and the dates of my travel.
NB: I used a Visa credit card issued by French bank.
Well, you don't know how good South Korea is in this domain. It is the country that is widely considered to be the highest credit card penetration in the world.
South Koreans have on average five credit cards, compared to two in the U.S., and the country has the highest credit card penetration globally. Consumers in South Korea also use credit more often. There are 129.7 credit card transactions per year in South Korea, compared to 77.9 credit card transactions annually in the U.S.
If I remember it correctly, the government tried hard to improve credit card penetration, including making shops that has over certain amount of sales obligated to accept cards, giving tax deduction for citizens using cards, etc...
In my experience, even all the drug stores, local cafes and cheap local foods accept my Amex, in Gangnam, Myeongdong, Dongdaemun, and Hongdae.
The only case you cannot use your credit card is on the public transportation; Well, you still don't have to carry cash as it accepts T-Money, but you must use cash to recharge your T-Money card.
However, it is not uncommon to be unable to recharge your public transportation card using credit card; all of Suica (Japan), SPTC (China/Shanghai), Octopus (Hong Kong) don't, either.