I've done retail as well.
And as a semi-serious amateur musician, people ask why I NEVER buy guitar things online. Simply put, I NEED to be able to see, try, and touch them before buying them, and I'm happy as hell to pay slightly higher prices to be able to do so.
Nobody can stay in business by being your emergency vendor, but smoke shops that are already in business could add a small but potentially worthwhile revenue stream to their already viable businesses.
The mall vendors charge WAY more than they should- a smoke shop shouldn't have much trouble undercutting them.
If it were my business, I'd probably start small. Research a GOOD starter kit (I'm thinking riva510) and put a dozen under the counter, one on display somewhere. Research a dozen or so very popular juice flavors, and buy them in popular strengths. (0?, 6, 12, 18, 24? ) Buy a hundred or so bottles. Make up a counter top display, or if space is limited, even a poster. Most shops have this much space to spare already, so you aren't losing any valuable retail space.
Most of us would be loathe to pay mall vendor prices for anything, ever. But it wouldn't be too hard, as a smoke shop owner, to offer the riva kit for $50 to $60, bottle of juice (30ml) for $25 to $30. There wouldn't be much time investment (place orders online at need, await delivery to place of business.) Markup is moderate- even KNOWING I could buy cheaper online, I'd sometimes buy at that rate, especially in a hurry. And I'd have been delighted to find those prices in a smokeshop for my FIRST e-cig.
Another option would be to try to establish an arrangement with liberty flights, FSUSA, cignot, or some online vendor to put their products and publicity in the shop.
THere's not a LOT of money in this- but there's not a lot of money in most retail setups anyway. There is a potential for a reasonable return on time/space spent in there somewhere though.
Remember- many of us still buy cigarettes, cigars, matches, ashtrays, snus, and other smoke related products, at least sometimes. (Either because we use them, or because someone else around us does.) I wouldn't go to a mall vendor just to buy juice- but if I had to buy cigs for my wife (just rhetorical, she quit), I'd go to a place that had e-cig things for me too!
Best
Ande