A ring of wishes is usually extremely expensive. In the range of 97,950 gp. It is not generally something you buy in a magic shop. A two-wish variant would be slightly less expensive, but still very expensive. If I encountered one in a magic store, for an affordable price, my first reaction would be: "What's the catch?"
So it would make more sense as a treasure reward, or loot from a powerful spellcaster. A good way to introduce it into the plot, would be to have a side story where the ring's previous owner is killed to obtain this item. But perhaps he hid it very well, or locked it in a safe, which the killer was unable to open... or so it seems.
In my own campaign I had a side plot revolving around a magic lamp with a genie, and the genie revealed that sometimes its owners don't live very long, because everyone wants a wish granting item, and there is always someone willing to kill for one. I also introduced a rule that whenever a wish is made, it has to come from somewhere. Wish for a castle, and someone loses their castle... and they may come looking for it. That was the dark side of wishing. In my campaign it didn't even need to be cursed. Just the knowledge of the dangers of greed, and knowing the abysmal fate of the lamp's previous owners, was enough of a curse to make my players very anxious to wish for anything at all.