Although your position is an interesting thought exercise and one I've engaged in myself, you're right it renders selfishness and selflessness meaningless terms. Meaningless terms are useless.
The rest of planet Earth takes selflessness to mean undertaking actions to help others that have no tangible benefit to oneself. Selfishness refers to actions that benefit you exclusively, often at the detriment of others.
What you're saying requires the terms to be redefined to fit a meaning they were never meant to have.
The definitions you just advanced would be nearly as useless, as almost no behaviour would ever meet either standard (unless we use an extremely narrow definition of tangible).
We choose things because we want to. That's sufficient benefit, I would argue, to prevent selflessness under your definition. The extent to which we take others into account is based entirely on our own desire to do so.
If we ask why enough times, eventually every answer becomes "because we want". That renders every decision selfish under your definition, unless you're somehow granting incidental benefit to others as equivalent to intended benefit.
If I do something purely for me, but it just happens to help others, is that a selfish act? Does that benefit to others matter at all when judging my actions, given that it ultimately had nothing to do with why I chose the action?
I simply don't think these moral standards have any basis.