Can I reuse chopped onions?
If you're operating a hot dog stand, I'm going to guess that you'll want to follow your local health codes.  There's a very good chance that it'd be a violation.

In most areas, it wouldn't be anything to stop you from using it for your own meals, so long as you didn't serve it to the public.  (and you might have to transfer it to a different container, so you keep the public &amp; personal stuff seperate).  So long as they've been kept chilled through the day, I'd be inclined to use them in some sort of cooked dish for dinner.  At worst, you get sick, but you don't get the public sick.