For some reason when I try to sell my cargo at the spaceport trade depot, I only get a message that there are no artifacts in my starship hold. It doesn't even give me the option to sell the many minerals I've gathered. Is this a bug or something I've overlooked?

In SF2 you can no longer sell stuff at the spaceport. You need to sell them to trading posts at various planets. The name is called Trading Routes of the Cloud Nebula for a reason.

Also note that even the various alien trade outposts are fickle -- the selection is semi-random. E.G. the Golgafrinchans might always be willing to buy Telephone-Sanitizing Soap and White Howler, but they rest of what they will buy on any given visit is random.

As a general rule, don't buy large amounts of anything unless you know somebody who likes to buy it. There's no profit in hauling around 200m³ of a commodity for months, always hoping somebody will take it off your hands.