This is most likely a malicious flash widget that somebody injected into the eBay web page, probably by buying an ad on the eBay platform.
Since GarageSale uses the same system libraries as Safari, installing an ad blocker in Chrome won’t have any effect.
I suggest you either uninstall Flash from your computer and install the “ClickToFlash” extension in Safari: https://extensions.apple.com/details/?id=com.hoyois.safari.clicktoflash-GY5KR7239Q