Currently a player is allowed by Rails to sell a share to raise capital to buy a share ... and subsequently to sell *that* share. Most groups interpret the rules to prohibit this behavior. Selling other shares is irrelevant as it could legally be done beforehand at the same effect.
I have changed this bug report into a feature request. The requested feature would be to add an option to disallow buy/sell of the same share. Such a feature already exists in 1856, so it can be implemented easily.
However, the question remains whether it is worth while to clutter the options list even more. The absence of the requested option does not prohibit valid play, it only allows play that some consider invalid.
So far one (positive) reaction has been received.
Changed to a low priority