I take quest from man staying near thief cave, destroy all sceletons and get ring from thief. Before i complete quest with ring, i go to town and sell some things. But accidentally i sell the ring. When i discover i dont have ring, the traders got new stuff. What now? How i can get ring back?

I think you cant.

You have 3 options:
1. Live with it.
2. Load an older Savegame (it is always good to save as often as you can :P)
3. Do cheat.

I find another option.

4. Kill the thief and take ring from him.

Now i complete the quest without cheating and load old savegame.