Horrible mattresses!! Don't waste your $ if you want quality

I am beyond upset that I spent $450 on a pillowtop mattress set, thinking I got a killer deal. It was super comfy in the store. I've had it about 6 months and the top is all lumpy and it's dipping already!! Serta and Beautyrest mattresses are usually good quality so I thought they were legit but I'm now wondering if they're the real deal or knock offs/errors or something. I had a $100 mattress in a box from Walmart that was more comfortable before this. I gave it away and wish I still had it. I may have to toss this set and start all over now as I wake up with backaches daily due to how soft and squishy this mattress is.
Also bought a twin mattress for 119, for my daughter. I told the lady I wanted a very firm mattress. She told me they were firm. I get it home and it's all wobbly when u lie on it, feels like you're gonna roll off. It's bouncy, like the springs are too springy or something. HORRIBLE QUALITY.
I ended up buying a used mattress from someone off FB for $30 that is firm and much more comfy. So this mattress is not being used.
Total waste of money!!
So in all I wasted over 590. on crappy mattresses that are horrible quality.
I have back issues and need a decent mattress. Guess I'm gonna have to go to a real bed store and spend $1000+ now after flushing my $ down the toilet at Mardens.
Take my word for it, the mattresses are NOT good quality!!
DO NOT BUY MARDENS MATTRESSES, unless you enjoy throwing your money down the toilet. :( :( Very angry consumer. I don't care what bargains they have there, I'll never shop at Mardens again