Sorry if this was already mentioned somewhere (or is a bug), I skimmed and tried the search feature but couldn't find something about this. If there already is, please disregard!
Anyway, today I noticed while adding some potions to SDBs that I could deposit both full and used potions into the same SDB. It seems this causes the potion(s?) with fewer uses to always be withdrawn last when removed (i.e. if you add one full, one used, and remove one, you'll get the full one).
I then decided to test what this would mean for SDBs in towers. I put two potions - one full, one used - into the same SDB and set a low price to get a friend to help testing. She first purchased both separately, then both at the same time.
Not only did the price in the SDB not change when the potion available for purchase (the second one) had fewer uses, she mentioned there was no indication to her that the potion she was purchasing had been used. The only information available was what you'd find at the encyclopedia - that it's good for about 11 uses (it was a Seed-Dibber Libation). Of course, when the potion was sent to her inventory, it showed up as having been used. Buying in bulk had the same results - she paid the same price for both potions.
This brings a concern: when buying potions, powders and such from towers, you could be unknowingly buying something that doesn't have the full uses for the price of a full unit. This doesn't necessarily mean the seller is trying to scam anyone - when depositing, they could just use "deposit all" without knowing/remembering they had something that had been partially used.
So... I'd suggest either having the price entered in the SDB be assumed as the price for the full number of uses (so the price would automatically adjust if an item isn't full - e.g. if you enter 1100 currants as the price and the buyer purchases something at 6/11 uses, the price is adjusted to 600 currants), or disabling the ability to put items with different amounts of uses left into the same SDB. Just having some indication of how many uses are left may not help since people often buy in bulk instead of one by one, so. :x
This problem doesn't exist in auctions since items with uses can't be stacked.
Sorry again if this had already been mentioned somewhere!
(If it hadn't, though, it'd be great if someone could help double check that this happens - I couldn't do so myself since my friend doesn't have a tower, but I did go to a random tower and purchased a potion and didn't see any specific indication of how many uses that particular potion had left before purchase, either.)