So while it's a fairly small square shape it's fairly open and minimalist, all the vendors follow the same kind of theme (white marble background to the images) and I feel it works very well for a low-prim solution.
I am however working on a new store design that I will be producing on a larger, dedicated plot of land, it's going to have a good 900 prims going for it so I can have a nice, big, open store but with plenty of little bits to explore as well. And without using mega-prims I hasten to add

I won't give any more away though, as it's going to be something pretty cool.
But I guess ultimately, what works for you will depends on a few things:
- Do you have a lot of stuff to sell? If so then you need to maximise the use of your space, in which case a big box is probably best. Just make sure to have plenty of space to move around and see things.
- If you have a few items (or a handful of flag-ship items) then you can go for the more 'intimate' smaller rooms which put the focus on the fewer number of items in them. It helps to have a central area to this though so that people can easily get to the individual rooms they're interested in, by category for example.
- Avoid multiple floors unless you're stuck for horizontal space. If you do then I would recommend ramps (spiral ones are nice) as these are just more fun to use than teleporters that require right-clicks (forcing people out of mouselook when they're browsing) and which make the experience less seamless. If you end up with multiple floors though then by all means provide a teleporter for people who know what they're looking for.
If you have some big items that you want to get a lot of attention then rez some examples in the middle of a room with easy ways to purchase them (do NOT tell people to go to a vendor, I hate that =). Less important items like accessories or other things that are more for fun but not really flag-ship products can go around the edges.
Try to arrange things evenly, a lot of the worst shops are a box which just have items arranged all around the edges with no real pattern, or worse, cramming more of the same into a space without thinking about how cluttered it becomes. The main-problem with this is that if they are all different sizes then the shop becomes uneven and people are put off. If you have big-items that you don't want to push then place these in vendors with similar items, or on their own if they don't belong to a category; big "miscellaneous" vendors are horrible.
For example, in my store all my items are housed in vendors, with demos that rez as required. However, since I will soon have the prim-limit needed I will be rezzing permanent copies in prominent places within my store, with a way to buy them directly. The advantage of everything being in vendors (ie the same visual size) though is that the store is "even", meaning people are free to find items based on the signs I have up describing the category. They can then use the vendors to browse that category easily, as each one represents a sub-category within it.
This is opposed to items vying for attention by having large items overshadow smaller, more valuable ones. Or just so much junk lying out that the good stuff gets overlooked, or the whole thing just takes plain ages to load.
In fact, if your store caters to anything that has a real-world equivalent, then find a few stores that showcase these in real-life. Even if you have no real-world parallel, just look in a range of stores and note how things are organised; things that they want you to see are given pride of place, things they want you to impulse buy are positioned near check-outs (places you are most likely to consider what else to buy). Consider also how some of these things make you feel as a customer, is the store too cluttered? How easy was it to find a random item?
Ignore the specifics, like shoes being on shelves, as these don't always translate well to SL (it means a lot of detailed items to rez), but on the sort of general things they do to organise their store, WHERE items are in relation to where you enter, how they layout the store so that you can find things. There's lots of interesting patterns if you look for them.
Okay, I know you didn't want an essay, but there you go, hope it's of use! The more well-designed stores we can get in SL the better, I hate product heaps and teleport nightmares

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