"Da" is not common in the US. If I saw it on its own, I would almost certainly assume it was the Russian word for "yes" before I drew connection to "Dad". Most people still say "Dad" without dropping the ending, or "Pa" if they were raised by heathens.
Also, when I first saw the image, I read it as "Yerda", which I assumed to be something like a female given name of some sort of Scandivanian origin.
I'm only sort-of aware of the background on this one, so bear with me:
Avon is a brand of cosmetics being sold door-to-door, so it became a favorite of many transvestites in the UK, many becoming Avon salespeople themselves.
So "Yer da sells Avon" could also be meant to imply "your dad is a transvestite".
Wait, would you really not cop that right away?