Always have the Steam Community Market checkbox checked using Tampermonkey? (or else)

When you buy something from the Steam Community Market, you always have to check the checkbox that says "I agree to the terms of the Steam Subscriber Agreement[...]". So when I try to get something cheap by constantly refeshing the page, then find a good deal, i always lose it because i cant hit the checkbox and the n the buy button again fast enough.

I want the box to always be checked. Can that be done with Tampermonkey? I not experienced with javascript, but i found this:

document.getElementById("checkbox").checked = true;

which seems logical to me. I put it in a new tampermonkey script and (i think) added steam market to the list of websites the script activates on. I thought maybe it will check all boxet that are on the page? Oh well, has anyone done this prior?