They sold the stocks out from under me.
Is this "sold the stocks out from under me" common? It seems to mean they defrauded me.
There is an idiom that employs the latter half of your sentence:
This idiom is quite self-explanatory: it simply means to remove some kind of vital support in a swift, surprising fashion. Imagine Person A standing on a small rug, and then Person B comes along and pulls as hard as they can—Person A will obviously fall over because the ground under his feet seems to disappear.
In the sentence you have here, the same kind of effect is being applied to someone selling stocks in a kind of semi-idiomatic way. Now stocks are naturally not quite as easy to be physically reliant on, but you can imagine them as forming the basis of some kind of (financial) support.
Say Person A (different Person A this time—the other one’s been mistreated enough) is in financial trouble, but has worked out some complex scheme to get back on track. To this end, he is relying on some stock that he has to use (as some kind of leverage, say) to make the scheme work properly. The stocks belong to Person B (same Person B as before—he’s a right plonker!).
So imagine that Person A has set his brilliant scheme in motion, and all is going well … until all of a sudden, Person B decides to sell the stocks in question. Seeing as Person A was quite reliant on these stocks, he’s very likely to suddenly feel that Person B has sold the stocks out from under him.
The only idiom close to it that I have heard of is:
under somebody's nose
if something bad happens under your nose, it happens very close to you but you do not notice it.
I'm amazed that it was going on right under his nose all that while and he didn't realize.
So what you could be saying is an abridged version of:
They sold the stocks right under my nose.
i.e. They sold they stocks without my knowledge, while I was there all along.
So "sold the stocks out from under me" is not idiomatic but as it a common situation, could be an oft used expression.