1. What does it mean?
An abbreviation of 내 돈 주고 내가 샀다 (“I paid for it and bought it myself”). It often appears alongside product, restaurant, and service reviews to say the reviewer paid for the experience.
2. The nuance
It is a claim about how something was obtained, not a guarantee that the review is accurate or unbiased.
3. Examples
“이 가방은 협찬 아니고 내돈내산이야.”
This bag wasn't sponsored; I bought it with my own money.
“내돈내산 후기라고 적혀 있길래 읽어 봤어.”
I read the review because it said the writer had paid for it themselves.
Sources
Korea Press Foundation: reviews and sponsorship disclosure
References support the meaning and usage. Example sentences are written for NoCapWiki.