(This definition is not about a word you can use in everyday use. A Sealandball is a character a user may adopt to be as, and is only that.)
A Sealandball is a countryball whose identity is the independent micronation of Sealand.
A Sealandball mustn't be confused with the countryball of Trinidad and Tobago, whose flags are similar, but are entirely different entities. Sealand's flag follows as Red, White, Black (left to right); Trinidad's follows Red, White, Black, White, Red.
A Sealandball wouldn't go into the classification of countryball, as it is a micronationball, or be differentiated to become a userball. But for the sake of simplification, it is a "countryball" - a political entity represented by a ball with two eyes.
To use, have, or be a Sealandball means you are well-versed enough in geopolitics to know about it, and for that, I applaud you.
"Which countryball will you represent?"
"I will represent Sealandball!"
"Oh, obscure!"