I've been checking seller stock on our Mags for about a year, and recent posts elsewhere prompted me to reveal the method.
On first check of a site, click their "Add to Cart" button. Then use their Edit function to Update to a higher qty, like 1000, 2000 or more and the web page will revert to the qty in stock and available for order now.
On subsequent checks, I still use a high number even tho I know last time is was a lower qty available. Esp. important on Walmart site cuz they get/got regular shipments of additional units, so the qty would often be higher than 5 days ago or so. Amazon and J&R only accept 999 max., and often they won't ratchet down below that for some reason.
I haven't noted it here previously cuz early-on, some sellers would HOLD that stock in abeyance until some later time, and I didn't want hundreds of people potentially screwing up their order systems.
I've since noticed that seller web sites now say they WON'T hold your qty forever, or they give you an option, so they appear to have fixed their open-ended stock-freeze potential.
But, please, remember to Delete or Remove your phoney qty before leaving the site, just in case?