According to G40 rules, if you can’t place all of your units, for whatever reason, during the place units phase, you get a cash refund and you get to choose which units are refunded.
Several different questions about this have been asked of Krieghund recently. He explained the history of Revised and AA50, and why it is now a cash refund (instead of carrying over to next turn as in AA50). Long story short, this system can be gamed hard, in somewhat rare instances.
It does not seem to me that the intent of the rule-writers was to allow for intentional over-buying and refunding. If I was playing you face to face, I would stop you in the buy phase if you were purchasing more than you could possibly place (however, there is no rule expressly against this).
The big advantage you can potentially get from over-buying, is that you can decide to refund certain units after seeing how the dice go. For example, buying carriers to make fighter/tac attacks legal. Another example is buying transports but only placing them if the threats to the new transports are succesfully neutralized.
I think it’s clear the intent of the rules is that you purchase units before you know how the dice are going to fall. This loophole allows you to effectively change your purchase after seeing your dice, which I think goes against the spirit of the game.
Default rule in league play is now officially this:
If you overpurchase, your OPPONENT will choose which units you are to refund.
This rule is needed for online play since normally a complete turn is finished before the opponent even sees it. In face to face play, the opponent would be with you phase by phase and would presumably stop you from purchasing more units than you could possibly place.
If you and your opponent agree, you may play that over-purchasing is allowed.