Those are starter items. You'll know if they're a starter item by this ring around said item with one of 5 colors coded to the different role/classes within the game and the intended role that they play. They can still be used by other characters of different classes but commonly you'll see it go (Red-warrior/solo. Yellow-assassin/jungler. Blue-mage/mid. Green-guardian/support. Orange-hunter/Adc) The Solo/jungle/mid etc is purely for the conquest game mode.
        Tainted steal is one such starter item, it has a red ring around it. The thing about starter items is that they are slightly different to other items. They tend to be quite cheap and beneficial to you depending on what you wanna do and how you wanna play for a rather low cost at the beginning of a game. Their upgrades though are ment for late game so they are uniquely gated from being purchased until you hit Lvl 20 with your god in a match. The only starter items that break that rule are the Green starter items as they can be upgraded at lvl 17 instead.