When the original Theros came out, I was really starting to play a lot of Magic and Gray Merchant of Asphodel was one of the reasons for that rise in play
Different stores have different techniques, but it generally comes to down to something like Mint/Near Mint (M/NM) being the best followed by Slightly Played (SP), and Played (PL), Heavily Played (HP), and finally Poor
That extra mana makes X-spells like Finale of Devastation and Genesis Wave absolutely devastating
Even outside of poison strategies, turning small utility creatures into real threats changes combat math immediately
Earlier this year, Ira Glass talked about how hard it is to start out in any creative endeavor, and while alteration is a bit different, a lot of it still applies: Nobody tells this to people who are beginners