There is no way a seller could know if a ring will turn your finger green. They would have to know the exact acidity of your skin and run an acid test on the ring to match. Even then it is still possible to get discoloration, even from high end gold and silver rings. Sterling silver and copper are 2 of the leading metals to cause discoloration. It doesn't mean the ring is cheap although it might be. If it happens with lots of rings you wear you should look into why your skin is reacting there are ways to lessen the causes and prevent it in a lot of cases. If you google it there are many articles and help with discoloration.