Just because you can buy something doesn't mean it works. You can buy a home gym, for example, but it won't make you thin. However, there are some things you can buy that won't work even if you actually try to achieve a result.
You buy devices that control pests using ultrasonic frequencies - they are purported to work for mosquitoes, cockroaches, ants and now there are new versions targeting bed bugs, since those were in the news a lot.
A new paper reports the results of tests of four commercially available electronic pest repellent devices designed to repel insect and mammalian pests by using sound.