Well, YMMV means Your Miles May Vary, but that doesn’t really explain anything. YMMV, basically, is just a disclaimer that means results may be different for every person. In terms of deals, it is stating that either the deal’s price varies by region, inventory varies by region, or the deal is only available to a select group of people.
One great tool that we use in YMMV deals is a site called brickseek.com. This site allows you to check your local store and see what the current price is. It also lists the on-hand inventory but due to theft, store inventory inaccuracies and other factors, I wouldn’t fully rely on their inventory numbers. If you are planning to drive a long distance to make your purchase, it’s always best to call to double check. The current stores that Brickseek monitors are Target, Walmart, Staples, CVS, Lowe’s, and Office Depot.