OK this may be a glitch or something but i had just bought 10 medicine about 10 mins ago and the silver was deducted from my stockpile but i didn't get anything in return! the sound played and the cargo pod landed in the beacons radius and there was nothing dropped off. i have both the coms unit AND the trading beacon, the beacon is outside smack dab in the middle of the stockpile i set up for it and the coms unit is inside. the stockpile is set to accept clothing, weapons, medicine and raw mats like cloth and hides so it should have deposited the medicine in the stockpile right?
only if there was space in the stockpile, and if there is no space then it should 'pop' into the nearest empty square to the beacon, and with clothing degrading when outside you might want to have an undercover space (with beacons) for the selling goods area, and use the outside beacon area as a RECEIVE goods area only (maybe with silver/gold storage)
the stockpile area that was designated as the drop off zone only had 6 items in it and 2 lots of silver. the stockpile is as large as the beacons radius if that helps.
also from what I remember, the medicine stacks for more than 10 so there might have been only one medicine visible, but it would have been a stack of medicine
IIRC this bug has been fixed in A13. If it happens again in A13 please report it.