Welcome to Fantasica and welcome to Fantasica-Unoffical Forums!
3*s do not go for 75k, probably 150k to 300k, depending on the card's stats, abilities, or whether it's a unit or monster.
4*s sell for 1 TE(Time elixir) each card, unless you get a good seller that sells more than one 4* card for 1 TE.
I would use a TE for a 4* card with good stats mainly, or a Knockback skill at the very least.
Good luck!