Is it legal to sell a custom element that provides a Technic pin connection?
I am not a lawyer, and I recommend you consult one before you start dropping money on injection molds, but as far as I know this should be fine. SBrick definitely already does this:



In most jurisdictions, patents have a term of 20 years or less. The Technic pin connection dates back to Expert Builder in 1977, so it is not patent encumbered at this point.

The two patents that you linked to from the late 1990s appear to be for ZNAP, which uses a different connection system: