When purchasing from the secondary market, exercise the same caution you would when purchasing regular Magic cards, or anything on the internet, for that matter
Formats were less defined by resolving creature and more by combos such as [casthaven]Prosperity[/casthaven] and [casthaven]Cadaverous Bloom[/casthaven], artifact mana and [casthaven]Phyrexian Processor[/casthaven], and [casthaven]Necropotence[/casthaven] and cards, or by decks full of every [casthaven]Counterspell[/casthaven] available
After all, there are tons of people building Commander decks with these, regardless of whether or not they're actually legal
And there are 25 legendary backgrounds: 5 commons, 15 uncommons, and 5 rares
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