while merchants accepting USD in cash appears to be uncommon, be on the watch for merchants who take credit cards and process the purchase in USD instead of won.

dynamic currency conversion adds a 3% fee on top of any foreign exchange fee, and the exchange rate it uses is often considered a poor rate. the merchant is supposed to ask if you'd like to make the purchase in USD (really, in your home currency, so this would apply to euros, yen, etc). but the reality is that the DCC fee is free money to them, so they might not be so scrupulous.