There's 4 versions of magic online:
MtGO: old official version, you can buy and sell singles for similar costs to paper
Cockatrice: Old unofficial free version. leaves a lot of rule/effect enforcement up to players manually.
Duels: "new" official version, never implemented complete rulesets, abandoned by devs 2 years ago, leaving collectors who paid good money SOL
Arena: new official version, but with hearthstone model of only RNG card purchases, no singles or resales.
Personally, after putting close to 4 figures into MtGO, then hundreds into duels for nothing (on top of the fortune I've spent on paper), the price fatigue has ruined arena for me, and I don't give them any money.