I'm sorry, but for those complaining about water quality, your aquarium is just a container. Be it a NanoCube, an Eclipse, a rubbermaid trash can, the water quality has everything to do with what you put in it and how you take care of it.
But back to the OP's main concern; He already has the tank and its set up. I dont think he can get nearly as much for it if he returned it now that its used. Theres nothing wrong with using it as a marine tank. All he needs to do is add a powerhead and as long as he maintains the tank, it should be fine. If he wanted better filtration, cut a hole in the back of the hood and add a powerfilter or a HOB skimmer if he was feeling adventurous. Problem solved.
If he wated to set it up as a reef, sure hes going to need to MacGuyver himself some better lighting but its not impossible and I dont think it'll cost him a fortune (especially considering he didnt pay for the tank in the first place). It can be done.