Warning: Massive Post Incoming!
I vaguely recall reading other posts that suggested the store have an inventory that is stocked/restocked each day, rather than entirely new supplies with no holdover from before. So, while this isn't exactly a new feature suggestion, I'm just voicing my support of it.
However, what I'd really like to see is some kind of special order system: paying extra to get a specific item that may be hard to find in stock. There are times when I'm sitting on quite literally a million gold and making profits of 10k a day (generally toward the end of my run) where I find myself spending days and weeks just checking the store on the off-chance it's stocking a particular Ring or Necklace or something to equip on one or more of my girls. If there were some way to place an order for the next day (or week or what-have-you), paying twice as much for the guarantee of the item's arrival, it'd save on patience.
Additionally, some kind of system to allow us to buy multiple of an item at once would be helpful, though probably difficult to implement given how the store is currently coded. As well as something to give a specific item to a group of girls, give a group of specific items to a specific girl, and/or give a group of specific items to a group of girls, as well as confiscate in the same fashion, and swap items between girls in the same fashion. This could be accomplished by using something similar to the system currently in use to transfer girls between brothels. If it looks like the Transfer screen, girls could be listed where brothels are, and the inventories of the selected girls could be listed where the girls of the selected brothel are. The PC would be at the top of the list of girls, and all of the girls' names would have check boxes next to them, to allow players to select multiple girls at once. I would imagine the Inventory screen would show the list of items that all of the girls have, listing how many in total. Additionally, I'd imagine the items themselves would have check-boxes, so that players could select multiple items at once.
Fixing the item distribution system is critical, IMO. As it currently stands, equipping an individual girl with a Sexy Dress, Pearl Necklace, High Heels, Hairbrush, and Hand Mirror, takes 21 clicks, and that's starting at the Details page: equipping a full brothel takes between 420 and 1050 clicks, depending on which one and assuming the player uses the keyboard to change girls. If you decide to upgrade that girl's Sexy Dress, Pearl Necklace, and Hairbrush to a Sexy Sheer Dress, Nice Jewelry, and Gold Hairbrush, while reclaiming the originals to resell, that takes 21 clicks as well, assuming you remove the original items first (to, say, avoid the hairbrush crashing glitch), again starting from the Details page. Item distribution takes far, far too long: the only way I've been able to pull it off is by equipping girls with the highest-end gear I can find, the moment I get them, and that's still a nuisance.
Finally, I can't actually tell what most of the items do. Some of them are pretty obvious, like most of the Consumables: but ones like Makeup/Perfume/Flowers, dresses, necklaces (that aren't Control), rings, and shoes aren't apparent until you start messing around with them, if even then. I've played through a dozen times at least and I still haven't figured out why I might bother getting a girl some Makeup. I've seen a few item mods that list the items' effects as part of the description, and I think that would be a useful standard to adopt.
So, uh, yeah. That's all my item suggestions. I'm not really a programmer, so I don't know how hard this would be to do and I admit I haven't been reading all the threads super-diligently, so I might be repeating ideas someone else already suggested: but... well, those are my two cents. ^_^;