I would say 'used, in good condition'. You can also add an age, if you know it: 'slightly used, great condition, about six months old'. There are also phrases like 'as-new', which means that it has no signs of wear or age at all. Perhaps you could look at some ads, to see some of the phrases that people use?

I'm not 100% sure that this is what you are looking for, but for example with used items on Amazon, the phrases "like new" and "perfect/great/good condition" are often used. If it is something that you have opened but not used, then "unused" can also work.