Legalities for disconnecting and selling our central A/C unit?

You need to be professionally licensed to remove and recycle refrigerant and compressor oil. You would have to incur a cost to have your units pumped down before you can remove them. The larger issue for you is who would want to buy the unit. No contractor will want it. If you did find a DYI guy he is only going to offer you a couple of hundred dollars at best. The refrigerant recovery cost is about the same amount. You really have nothing of any value so just leave it in place and don't decrease your property value any further.