That animal is in very poor condition, no doubt about it, but the caging right now is not likely the source of the problem. Meller's and many chameleons often arrive in very poor condition when wild collected (especially relatively cheap ones). Chameleons in particular are usually severely dehyrated. Getting them back into a healthy state is not easy, and in some many cases impossible (e.g., if there is permanent kidney damage) and takes a degree of skill. This animal was likely in terrible condition when it arrived, and the caging conditions--which might be just fine for some chameleons in a healthy state--do not fit the bill to get this animal back into good health. Glass is fine in most cases, but more cover and surfaces to drink from (i.e., plants) would be a very good idea, along with intense attempts at hydration and feeding. A healthy, hardy animal might be ok in a setup like that. A fragile animal in poor condition (like the one pictured) is almost certainly doomed.
I might talk to them gingerly, to see if some improvements can be made. You don't want to be combative though--that will make them blow you off. You want to stress that you are a happy customer and want to continue to be a customer and want to help them improve the condition of this animal.
Buying the animal is about the worst possible thing one could do. Firstly, this animal, even with appropriate care, may well be too far gone. Buying it transfers the monetary loss from them to you. In fact, usually the situation is even worse than that: animals that sell are replaced. That means that not only is this animal likely to die, but it will soon be replaced by another which will also die. Buying this one animal is a great way to kill two. We've seen exactly this scenario play out here recently. "Rescuing" an animal by buying it (for more than maybe 10% of retail value, at least) is always a terrible idea--it doesn't actually rescue the animal, and results in additional animals suffering and dying needlessly.
cj