'셈(을) 치다[잡다] is a very popular idiom derived from the noun '셈'.
(주로 ‘-은/는/을 셈 치다’ 구성으로 쓰여) 미루어 가정함을 나타내는 말.
'셈' is the noun form of '세다' which means to count or calculate.
돈을 세다. Count money.
참석자의 수를 세다. Count the number of
"어떠한 사실을 가정할 때 사용한다" means
You hypothesize an action as if it happened when it actually didn't
happen. (It is very similar to the subjunctive mood in English)
진수 엄마, 핸드 믹서 샀네. Jinsu Mom, you bought a hand mixer. (I notice you have a new hand mixer)
네, 외식 (eating out) 한 번 (once) 하는 (do) 셈 (count) 치고 샀어요 (bought). Yes, I bought it with the money I could have spent if I had eaten out at a restaurant once.
In order to understand the second sentence, you need to understand that the two actions (buying a hand mixer and eating out at a restaurant) would have cost the same amount of money. That's what '셈 counting' is supposed to mean. You count (calculate) how much would cost to buy a hand mixer and how much would cost to eat out and if you think there is no difference, you take one action (to buy a hand mixer) over another action (to eat out).
It is not an easy idiom, but the more examples you read, the easier it will be for you to understand.
다른 식당도 없는데, 그냥 속는 셈 치고 한번 먹어 봐요. (Literally) Since there are no other restaurants, let's just eat at the restaurant as if we were cheated by it.
This sentence is a little more complicated. "속는 셈 치다" is a broadly used idiomatic expression where 'being cheated = eating out in the first example" and "eating at the restaurant = buying a hand mixer". In other words, 'being cheated' doesn't happen, but the speaker thinks it is OK even if (s)he were cheated by the restaurant (even if the food were not delicious).
"속는 셈 치고 XYZ 한다" could be understood as "we would have nothing to lose even if we did XYZ". The second example could be translated to:
We would have nothing to lose even if the food were not delicious at the
restaurant because we already factored in the possibility when we made the decision.