First of all, there’s be the legal question of ownership. Although the place was abandoned by the British navy, you could be sure that a potential sale would result in a legal challenge by the government.
If it were decided that the place really was the property of the Sealanders, they’d be free to sell it - but whether it’s a sovereign nation is very much in the eye of the beholder. See Cecil’s column
That was in the 1960s - since then, territorial waters have been extended to twelve miles, which encompasses Sealand. BBC News | UK | Offshore and offline?