Hello 아진,
If you are not able to find a specific app in the App Store, it may not be available in your region. This article explains how to download from the App Store: Download apps and games on your iPhone or iPad - Apple Support.
Looking at that app on the App Store, it shows that it requires an iPhone, iPad or iPod touch running iOS 12 or later. If your device matches those requirements and you feel it should be available, you may wish to check with the app manufacturer by going to their website: OfferUp.
We hope this helps. Let us know if we can assist with anything else at all. Take care.