Aside from the obvious value as a tax evasion, gambling, and other Internet no-nos haven, Sealand can both serve itself and its clientele in an interesting way:
Open a duty-free shop and charge anyone who docks a "ballast tax". That is, access to the duty-free shop is based on a ship's dumping some of their ballast overboard at the base of Sealand. If they wish to keep their ballast, then they can drop any sizeable weight, say, a 1-ton boulder.
This is not as ridiculous as it sounds, as Sealand is located atop a sand bar, and with even 100 such boulders, it would be perhaps the only island at sea that is growing in size without the help of a volcano.
And the larger Sealand grows, the more it starts to make sense as a nation.
You've been living on top of that volcano too long...