I’ve recently purchased some car decals.
How recent is "recently"? PLEASE give more exact information when asking questions. Everything on eBay depends upon exact time limits.
To return an item as not as described (including faulty) you need to open a case under eBay's money back guarantee within 30 days of the date of delivery. Your only problem may be that after removing the faulty decals you should have kept them to return - your seller could insist on this.
By the way, why not try to leave some positive feedbacks when sellers deserve it? It looks so awful when your record of feedback left is just a wall of negs. It may put some sellers off selling to you.