It is absolutely, 100% legal for this person to keep the sonde, throw it away, or sell the sonde. This by far is not the first time I've seen balloons, sondes, etc...on Ebay and it surely won't be the last.
As soon as the equipment is let go by the HMT/meteorologist into the atmosphere, it is no longer government property. The return address/baggy on the box is simply a courtesy to allow the NWS reconditioning center in Kansas City to recycle the instrumentation for second use. Since the battery cell and elements often corrode the instruments, rarely are they retrieved in sufficient condition for this to occur. It also serves as a "gee whiz" factor to see where the balloon ended up. (From a meteorological perspective, it's interesting to note that we plot upper air observations in a vertical sense, when in reality, the sonde can be tens if not a hundred miles downstream by the time it reaches those 500 to 250mb charts). I once launched a balloon early in my career at Springfield, MO that was retrieved outside Pittsburgh, PA after it got caught in zonal 170kt + jet streak in the dead of winter.
More often then not, when these are called into the WFOs and their condition simply doesn't warrant the effort, we give the finder the option to mail it in, or keep it as a momento. Sometimes the mail bag doesn't even survive the 100,000 ft fall, and we certainly wouldn't charge someone to mail it back to KC.
Please consider the fact that on average, less than a half dozen of the estimated 730 launches a year from a given location are ever located. Most end up in the oceans or in unpopulated areas, which is why the contents are biodegradeable.
In short, there's no need to be a vigilante here. Let the guy make his $10 and move on.
Evan