I avoid it by buying refurbished mattresses from local charitable organizations.
I've never bought a mattress online, but I see little reason to think those guys are any better.
It's essentially no different than newly built. I've been buying these for decades and have never had an issue with bedbugs or any other nastiness.
They are less expensive, but not dramatically so. But I get a quality bed, don't have to deal with mattress shops, and get a little bit of feel-good from helping to employ people who otherwise have difficulty finding work.
And they deliver.
When I first investigated these things, I had asked them how, since they're only keeping the wood and wire, they can sell them at a discount. What they told me was that first, the beds they're refurbushing were originally donated to them, so cost them nothing. Also, since they employ people who are otherwise unemployable, they pay minimum wage, which keeps labor costs down. And thirdly, the government considers them a nonprofit work creation program and provides a certain amount of subsidy for the wages.
Definitely better for the environment though, and good for society if it means they're able to employ the less abled to help do the work.