Basically either a new command or in /shop, where you can sell your mob spawners for the same amount they originally cost so that players can either swap or sell their mob spawners to get different types.

 

Support, but I think they should give back only half or 3/4 of the money to buy them when sold

 

They shouldn't be sold for full price. Perhaps half.

 

Like @EccentricPotato_ said, maybe 3/4 of the normal amount.

Neutral

 

Neutral, duplication glitches allow players to get vast amounts of money very quickly. You can never be certain that they have all been patched