My container (used for land-based storage) has obviously seen travel, and has nothing beyond a couple corner air vents. None of the other containers I'm familiar without here (they're quite popular for storage) have anything of the sort. That valve is a nice idea, but doesn't really exist in most of the fleet of containers on the ocean.

They are, indeed, a hazard for other boats, though they generally won't "lurk below the surface." Either they're floating from buoyancy of what's in them, or they sink. The increasing water pressure as you descend makes "floating below the surface" a particularly unstable place.

Perhaps some new ones do, but generally most containers have nothing of the sort:

"Depending on whether they are full or empty, and on the nature of the cargo inside, containers may float at the surface for several days or weeks prior to sinking. Containers are not generally entirely watertight; while an empty container is likely to sink due to water ingress, a full container will likely float until air trapped in the cargo has escaped.’"

From the article, it seems that container losses don't generally happen one at a time, but rather you have some event where a ship loses hundreds at once. I guess if you're quick maybe you could get a ship in and scoop them up before they sink, but in order to save more than a few you'd need a big ship and most of the time you'd just be sitting around waiting for the next container loss event, and the ship would sit idle for months or years.

I'm assuming that if it sinks in deep water it's gone, but even there maybe there's an opportunity to salvage containers off the sea floor with the right equipment. Probably not worthwhile economically, but who knows?

Maybe the best case for this could be made for ships that normally at sea doing other things to be ready to divert from their ordinary tasks to collect containers if they don't have anything more pressing. (Coast Guard ships or Navy support vessels perhaps?)

Selling on eBay is theft. The contents if those containers belongs to someone. You can possibly make a living recovering the container for the owner, but if you don't agree to a price beforehand the lawyers will make a killing in court deciding what you should get.