Since you will be using these conditions to describe a variety of weapons, armor, and accessories, the adjectives need to be applicable to all three categories.
That keeps it simple for you, and keeps it simple and consistent for whoever is consuming the story you're creating.
With that in mind, here is my proposal, though I may be returning to make edits.
100% - Pristine
90% - Unblemished/Undamaged
80% - Durable
70% - Tested
60% - Blemished
50% - Worn
40% - Damaged
30% - Weakened
20% - Fragile
10% - Compromised
0% - Unusable
I'll admit, I want a better word for 90%, but I was happy to keep these concise and digestible, all at one word apiece.
Take what you will, ignore what you must.
Also, courtesy of Easy Tiger's comment below, here are some meaty options that more than deserve to be considered: scuffed, tarnished, battered, worn-out