Not only is used skeevy, but not all mattresses are the same. Some are too soft (which I hate) and some are like sleeping on the floor. I ended up getting a "cushion firm" which I love.
Also, it's an investment. Cheap mattresses don't last long, and they tend to dip/bow in the middle over a short time. A used mattress might look new, but you don't know how long it's been used or how it's been used (such as the weight of the people sleeping on it, which can result in wear and tear on the springs that you cannot see). Years ago, I bought a new, mid-priced mattress that was on sale. I thought it was a real bargain. Within a year, it was sunken in the middle and we found ourselves in the middle of the bed every night because you'd kinda just naturally "roll" there while sleeping.
You should both visit mattress stores, lay on the different mattresses, and decide what feels most comfortable to both of you. I learned the hard way that softest/loftier is not better. They feel comfortable in the store, but after sleeping on them for hours I tended to toss and turn. When you settle on a specific brand and model, research that online to see if you can get it cheaper.
That's exactly what I did about 5 years ago. Ordering online, I got a top of the line Stearns & Foster for literally half the price that I could find in stores. Here it is 5 years+ later and there is no sign of the mattress bowing in the center. It was WELL WORTH the money that I paid for it, and no more tossing and turning. And better yet, free delivery, setup, and removal of the old mattress.
If you buy used, you will have to do that all yourself.
I also don't like the Tempurpedic/memory foam mattresses because they sleep hot. Especially when it's warmer outside.
Hope this helps a little.