I was away for a while, just returned, and I find the interface when I go to the Market is completely different. I understand most of it, the Buy All button is very nice, thank you. But I had a hard time figuring out how to equip boosters.
I click on Boosters. I see the available stock on the left, the stuff I have in Inventory on the right. There are buttons to Buy, Sell, and Restock, but that's it. I had to click on the round blue button with the white exclamation point and read the instructional blurb very carefully to find out that boosters are equipped on the My Hero screen. Even after arriving at the My Hero screen, I had to look hard to see that there are in fact two other tabs to that screen.
I have it figured out, but I would respectfully suggest that it's far too difficult to find. If people can't figure out how to use boosters, you can't expect them to buy boosters.