Most people dont want it or like it
In decks that are playing a lot of Auras and/or Equipment, Halvar gives them a massive boost to give them double strike
Theres a stark difference between selling through a platform like Facebook Marketplace and meeting a stranger in person, or going to a local game store or convention and selling to a professional
Note that the power/toughness adjustment happens upon resolution of the spell, not when the creature returns to play
+1/+1, trample, and haste all on one piece of gear means your threat hits harder, gets through blockers, and starts to deal damage right away, so it snowballs way too fast when youre already chaining spells
Such cards will often take an entire turn to cast, or put your opponent multiple turns behind the curve