I intentionally disabled selling of magazines in SPON Money (you can still sell one-shot "magazines" like grenades). You absolutely can't tell how many rounds you have in a given magazine, so at best, you could assume that it had only one round in the magazine and thus sell it for buttons (e.g. 1/30 the price for a 30 round magazine, modified further by the resale markdown). I didn't enable selling for this meagre sum, since I knew people would just complain about that instead! I did overlook the problem with this, which is that you can't easily get rid of your old magazines when you upgrade to a new gun; you probably end up just dumping them all at the feet of the shop worker!
And no, the ammo command doesn't help since it only gives you the amount of ammo loaded into the weapon. If your gun isn't loaded, you wouldn't even know the number of rounds in any of your magazines. Of course, if you have more than one magazine type that could be loaded into the weapon (ball and SD, for example), you have no way of knowing which is actually loaded!
As Loki said, SPON Money is very much on a back burner. I'm concentrating on other projects and am already overburdened with trying to update them as it is. Yes, when it is finally updated, I will allow selling of magazines, always assuming that only one round remained, just to allow you to dump them out of your inventory, rather than to allow you to recoup the true value.