It depends on the specific open source license you're talking about. It remains as *your* program, you can probably sell it if anybody wants, but you need to keep the source available for free distribution in the GPL. Other licenses have differing terms. Some just require you to cite the source and you can do whatever you want with it.

Of course, all this is purely ethical and voluntary, if you decide to sell it for a million bucks, some people are going to call you an @sshole, but nobody is going to come knocking on your door with the police.

That's a different story. Excel scripting is still a form of intellectual property, and as such should be sellable. However, do look up your previous job contract. Some of them have a clause in which all IP you produce belong to the company, in which case they'd be justified in suing you if you try to sell it.