It's a fraud. Sealand is a small 50-year offshore platform 10 kilos off the British coast; and they're letting it go for half a billion bucks!
For $10 million bucks I've built modern offshore platforms that could resist typhoon-force winds, towed them a couple hundred kilos offshore (beyond territorial and contiguous waters), and permanently anchored them into the seabed with 100 meter-long steel piles. Did plenty of this in the Gulf of Thailand for offshore oil and gas drilling companies.
For a few hundred million bucks, I could do the same, but with a massive platform that could house dozens or hundreds of full-time staff, tow it out into international waters, and into water much much deeper. Heck, for a bit more, I could throw in a few machine guns and a cruise missile or two to defend my status as an "independent state."