Neither the Marlin nor the USP are a problem. You could sell them to anyone who could legally buy them, but the transfer must go through an FFL, who will perform a background check on the buyer, hold the gun for 10 days, etc. You could also sell the guns out of state, but again, you must be sending the guns to an FFL, who will handle the transfer for you.
Your MAK90 is another story. It was required to be registered with the state by 12/31/2000, and is illegal to possess if it wasn't. It also can't be transferred to anyone in the state. I would remove it from the state ASAP if it isn't registered. You CAN legally sell it to an out-of-state FFL, but it's against PRK law to offer it for sale to anyone else.
As far as market value, figure 60-80% of new cost is the RETAIL value of the Marlin and HK. If you sell them to a gun shop, they're only going to give you WHOLESALE value, which is going to be much less. To get retail price, you'd need to find buyers yourself and arrange the transfers through their FFL. The gun auction boards, such as GunBroker and AuctionArms are the best places to do this.
-Troy