Personally, I wouldn't care one bit. If I sold it to him, that means I thought I was getting a fair price for it. If he can turn around and sell it for more, good for him. Also, there's nothing misleading about using the word "custom". That's exactly what it is. He never claimed he built it himself, not that it would make a difference. As for saying "built by a microbiologist", close enough. Anyone shopping for an aquarium will recognize that as a meaningless fluff statement anyway. If selling things for more than you paid, or talking up your product were wrong, there would be a lot of people out of business.