Sam, just looked up the Rambam but it’s the other way around. Selling only helps if he is selling it to a Yid who he knows will melt it down. Moreover, it can only become Batul by an Oved Avoda Zara. The fact that the general population stopped practicing Avoda Zara doesn’t help the individual idols or their utensiles. The available idols at the time they gave it up would become Batul if some physical defacing took place.
Here’s a chance for a good Indian Bittul AZ startup. Just logon and for a small monthly fee we’ll be Mevatel all your artifacts. Disclaimer: it must be done before you buy it.