I would contact him and say the only outcome that is satisfactory is a full refund immediately. If he balks, speak calmly and explain to him that no other option is acceptable and this is the only time you will offer him such a simple solution. If someoene sells you something and conceals damage that could lead to product failure and your injury, that puts them in a seriously precarious position. He is aware that by putting a sticker over the damage, (and we all know that is exactly what he did), he made an attempt to deliberately conceal this damage. It would not be a common practice to start peeling stickers off of a product as an interested buyer so there is no way you would discover this on a pre-buy inspection. I would not go to his office. He will twist it into some kind of threat or intimidation and will likely turn the tables on you. If this fails, personally, I would hire a lawyer and possibly take him to court even if it cost more money. Why? Because I hate crooks.