I second or thrid or 4x idea of small, low overhead, space. My local scooter shop carries new and used Piaggio-Vespa-Aprilia (bikes & scooters) + -Kymco's. The showroom, accessories, and sales counter is very small; it holds about 3 bikes and 3 or 4 scooters - probably less than 500 sq ft. It is sort of boutique like, and almost crowded, but it, along with some choice historic Vespa decor gives you a enveloped in all things moto feeling. The rest of the inventory is in a big enough low fi basement just below, next to the good sized service area with 3 wrenches working.
The used scoots and some new scoots are wheeled out each day from below and placed along side the shop, still visible.
The area is in a higher rent zone, and he does well enough that he also has another similar shop 20 miles away in another high rent demographic - and oddly enough it apparently works well enough to keep them separate. Basically, even in a ride year round good scooter market like where I am I know he really breaks even, at best, on selling bikes; not surprisingly its service and accessories that keeps them afloat. Also, I would bet a Vespa franchise could be a pricey entry. The other scooter shop in the area is a big plate glassed ex car dealer about 9 miles away, also in a good demographic but with lower rent. Its all Chinese scoot shop, I don't even recognize the brands (Lance?) - although it appears to be about 4 or 5 different brand. They have garish paint and slogans on the windows which just adds to my reluctance to even go in to check it out for curiosities sake.
If you are in a college or university town it seems one ought to carry some low priced 50cc scoots. I am near a major university and they make up about 50% of the all bikes parked near the university, probably several hundred on any given day.