I bought a 2nd hand fridge/freezer and it's about 10 years old. The fridge is on top and freezer is on bottom. The freezer works fine however I found frost was generated inside the fridge around the back wall and right corner. The door seal seems OK and the temp setting is not too low. Before frost appears I notice the back wall and right corner surface are quite wet. After 2 days frost was generated on the surface and Over time more and more frost is built-up. I just wondering why there are water/moisture inside the fridge and how to avoid the frost? I haven't store anything inside the fridge and didn't open the door often.

Your answers will be appreciated!

Check you seal carefully. Sure sounds as if is is not sealing properly