It could be that the animal is septic. The good news is that Marine toads are usually extremely hardy. Once placed in a healthy environment, and a fecal is taken, you may have hope to save him. Do you have a good vet in your area for herps?

I would Buy a nice healthy one  your setting yourself up for large vet bills and heartache

Unfortunately, I have to agree with Mike here. By buying the animal you are only contributing to the pet store's abuse. Now if they were open to giving him to you, that's another story. I'd try to save him if he was free, but when you buy the animal you aren't rescuing it, you're buying it. They will just put a new one in the same conditions while profiting off of it at the same time. I hope this poor guy gets the care he needs regardless though. I'm not well versed in amphibians so I can't really give any advice on whether or not he is saveable or not - but from the picture he doesn't look too far gone to me. 

i agree with mike and abi my local pet store takes in animals and cant resell them then they give me the reptiles so far ive had some success with 2 of the reptile ive recieved from them . if they offer you it for free and are able to get them back to health its a great feeling.

marinus is a super tough species....  just ask the aussies lol

Thanks everyone. Emily, I do have a very good herp vet in my area, so that'll give him a better chance. And I agree with everyones' reasoning when it comes to not supporting the pet store. It's another one of those stores that's a good aquarium, but then they decide they need to start selling reptiles small scale and have no idea what they're doing. Luckily they know me pretty well because I'm also really into fish keeping so hopefully I can negotiate something with them (then boycott their store). And Mike, I too understand where you're coming from here. I normally avoid pet store rescues just based on past experience, but I don't know what it is about this guy. If I can save him I have to try. Thanks again!

And Joe, excellent point.