First and foremost, welcome to ARC
Here's a bit of perspective from one who spent MANY years on the seller's side of the counter... please don't read me as harsh - I'm not, I promise. I am going to be rather matter-of-fact though. You've learned a very important lesson in this experience, so my post is intended to help you avoid a repeat, because it can and will happen again unless you're prepared to avoid it
First off - trading back to the store for store credit: You will only be offered a fraction of what the fish cost at retail. That is how stores make money, we buy at wholesale, and sell at retail. I know some people think that's evil, but that's how the world works. That's what pays the rent, the employees, keeps the lights on.... and that's the same math that applies to EVERY business on the planet. Many stores have a livestock sales are FINAL policy so if they offered to take it back, even without credit, you're lucky because I know of a handful of places that livestock is exit-only. Why should they offer you more for it, than it would cost them to buy another one from their wholesaler? Rather than being miffed that you only got a bit of what you paid for it, be glad that 1) They would take it off your hands, and 2) They gave you something back for it. They are assuming the risk that the fish will still be healthy and its upkeep until somebody else buys it. So while you might perceive it as a bit of a ripoff, I'm politely suggesting that there was more goodwill in the gesture to take it back and give you some credit for it, than you might have thought. I'd call that one in the 'plus' column. Being an ARC member makes no difference - in fact... some shopkeepers - even sponsors - don't like ARC members because we tend to be a cheapskate bunch
So that is not a plus for you other than presenting proof of membership for a discount at the register.
Secondly - you bought it, it's your responsibility. I really do say that respectfully. At the end of the day, the buyer is the one that makes the choice to purchase or not purchase and nobody held a gun to your head to do it. That's a huge issue in this hobby - impulse buying without having all the information about what you're buying, before you buy it. After all, it smiled at you, didn't it? With those blue glazed eyes, and ET grin... nobody can resist a puffer.
They're adorable. They learn to recognize their keepers and will spit water at you and wag their little tails - they are like dogs with fins. Nobody will blame you for being drawn to a puffer, not me anyway.
Having said that, anybody who knows me and how I ran my business, will attest that I would spend just as much time un-selling something that was likely to be a bad choice, but it appears that I, along with a very few others, am the exception. Many hobbyists liken LFS salespeople to used-car salespeople, we're out to get you. Best defense? Educate yourself. Pick up a copy of Scott Michael's Pocket Expert Guide to Marine Fishes for $20-30 and bring it with you to the store. I used to have a well-worn copy on the front counter and everybody used it. "How big will this fish get? What does it eat? Is it too big for my tank?" While there are a few bits of info in that book I find questionable, over 95% of it is spot-on. Then you can check an unbiased third party for the information without either buying blind, or relying on somebody who may or may not be as experienced as you think they are. Before I started working in the trade I had that book and Julian Sprung's "Corals: A Quick Reference Guide" and I'd take them to a store with me so I could research. Of course nowadays with a smartphone you can pretty much do online research right in the store. Do it. Save yourself a lot of aggro.
If the puffer was polluting your tank (I am guessing he was about golf-ball sized?), then you were likely over-feeding it. While it isn't a suitable long-term inhabitant, a small specimen would do OK in that size tank for a little while - but you wouldn't be keeping corals, or inverts with it, or even smaller fishes as those are all menu items. That and sum total of fishes for a small tank was over-stocking, IMO but again - that comes down to doing some research first.
At the end of the day, stores are in it to make money. Sometimes people get offended if a store clerk or owner suggests that the customer's choice may not be ideal (I've had my share of that too - respectfully suggesting that something was a bad idea, only to be told that the buyer knows what he/she is doing, so shut up and bag it up). I'm not suggesting you did that but plenty of LFS staffers just won't question anybody's choices, so maybe the person who would have benefited by that conversation, never get to have it.
We are here to help. I've made just about every mistake there is to make in my 25+ years in the hobby and trade - as a group here we try to help people get past their mistakes, and help prevent new ones, so I'm glad you're here and I hope we can be of help going forward.
Jenn