Sellers typically use baggies (the sandwich kind with the open top begin more popular than zip lock in my area) because they don't keep stuff around for very long, so no need to store long term.
If you are someone that stocks up because they don't come around very often, meaning you buy a lot for use over the course of a year or so (yes people like us exist) I've found the following to be a good solution:
Get a large bowl with a pop-on lid (the kind that fits tightly and doesn't let air in or out), one or two silica gel packets (those things that come in products to absorb water), and a large screen.
Place screen inside bowl, leaving a small space between it and the bottom, that space is where your silica gel packs will go (I change these often but I'm not sure its required). Your mushrooms will rest on the screen. Store in cool/dark place and they'll be good for a long while. I let fresh air in about once a week and change the packets about once a month. This keeps them nice and dry and as fresh as the day I got 'em. Its the best storage method I've found.
Now if you're consuming soon after acquiring plastic baggies will serve you fine and my method is extreme over kill.

Also, after a couple of months you can lose the packets if you want, I've gone months without using them and they stayed fresh. The packets are mostly there because my sources were bad about getting them as dry as I liked, so there was a bit of water left in there that needed to be pulled out before begin able to store them long term. Finally, where I live we have really humid summers, so I use the packets in the summer to combat the humidity. Again, a lot of this may or may not be required, I'm just telling you what worked for me in the past.
