They estimate based upon what these people (or those who work for them) tell them / self report to them, and also based upon publicly available information such as SEC filings of shares owned of publicly traded companies.
Possibly, in general, but in the specific case of Jordan, his spokesperson has declined to comment on his wealth or confirm Forbes' estimate.
Him earning $80 million a year doesn't mean he's actually worth a billion or even a dollar. Annual gross earnings can have little or no actual correlation to net worth.
But that's hypothetical and unrelated to your original point about him having money troubles. If he's in such bad shape that consistently earning upwards of $50mil a year (and being guaranteed to do the the same this year) still results in him being penniless, then the $15 mil he would get from selling just one of his unoccupied multimillion dollar properties is not going to make much of a difference.
I invest in companies and help others invest in publicly traded companies for a living. Believe nothing unless it can be verified with some reasonable degree of accuracy such as SEC filings and audits done by very reputable accounting firms.
We're not talking about due diligence on a company I'm about to purchase. We're talking about whether Jordan chopping the sale price of his home suggests money problems. Considering all of his other holdings and all of his other potential revenue streams, it doesn't.
What is the source of the Hornets' valuation?
Forbes and some other biz journals estimated that price. Of course, once an actual sale takes place, the actual sale price is subject to change, but the issue was never Jordan's precise net worth, but whether or not ONE of his home's sales price was indicative of financial problems. Just to purchase an NBA team , even if you're Michael Jordan, requires profound financial stability.