He might be assuming that people will make lower offers than the asking price, and some will NOT buy it if they don't "get a discount". There are people like that. If he said $300, they would insist on paying $200 or walk away...but if it is priced at $500, they will pay $300 for the exact same item. Silly :screwy:, I know, but there are people like that!
Some stores use this practice all the time, marking prices and X-ing them out with a "discount" price written underneath. The goods are never sold at the fictitious crossed-out "original" price... or maybe they were set out for one day at that price, just to justify the sign. Stupid, but sometimes it works.
Alternatively, he might just be trying to eliminate all the time-wasting lowballers calling to offer him $50 or $250, that he will never accept. If he just priced it at $300, they would call; with this approach, they might not.
He'll get a caller --probably most of them-- saying, "Look, I'll give you three hundred, not a penny more," who may be told, "If no one pays more within X days, it's yours". Some wont' like that and back out, but as a strategy it might work for the seller.
And then he might get lucky, and have people who really want those speakers but who don't read the ad completely. They'll pay $500, or perhaps offer a little less.
Basically, he has started the equivalent of an eBay auction with a starting price of $300 and a "buy it now" price of $500.
On balance, I agree that it's a stupid approach, but not completely stupid.