Could it be that the sales volumes are too low, which is why there are so few of them available for sale ?
I.e. They may need to make a huge quantity of them, to get very good volume price discounts, so that they can buy the components at nice high volume, reduced prices and get the bulk manufacturing costs down, to such a low price point, of $5.
E.g. They need orders of 250K, 500K or even a million of them, to economically viably make them, and have not been able to get such large (guaranteed) orders yet.
I.e. It is a bit like an Integrated Circuit manufacturer (such as Maxim) to stop making a new Integrated Circuit, because of a lack of sales success in the market place. I.e. The sales volumes are way too low, to economically continue trying to sell and make the new chip, so they abandon it.
It is also possible the selling agents profit margin on a $5 item like that, is too low (or even non-existent), to make it worth their while, selling them in bulk.
As other(s) have said in this thread. It is strange that they don't sell them at a higher price (e.g. $10). But at least have them in stock and available for customers to buy as many as they want (i.e. NOT limited to 1 per customer).