For selling it, bring it to your avenger/moc and in customization menu there will be a sell option. You cannot sell it if you don't have an avenger/moc whit vehicle workshop.

Can the APC be sold?

Can you sell the APC Tank in GTA Online? Yes, you can sell the APC Tank at Los Santos Customs for a resale price of $1,855,350 (60% of the original purchase), plus 60% of the value of your upgrades.

Can you sell Warstock?

You cannot sell vehicles that are obtained from Warstock Cache and Carry which require Pegasus for delivery, nor can you sell vehicles obtained through glitches or exploits.