The problem with GA is that it is almost totally devoid of modernisation, which makes running a shop difficult because everybody in the business is flogging more or less the same old junk.
I started flying in 2000 and the contents of the average shop (e.g. Transair) is almost the same today as it was then. They seem to live mostly on PPL and ATPL training supplies; the one thing which is sure to be in constant demand because it goes officially obsolete every year. The only significant thing which has changed is a few new GPS units but they are so relatively pricey that people shop around online. Garmin in particular seem to be pretty strict about retail price maintenance; it is illegal (in the EU) to do this overtly but there are "ways" to make sure it happens.
So, most pilots have a pretty good idea of what they need and I suspect most buy it online. I certainly tend to.
I'd go for an online shop every time, but selling some innovative products imported from the USA e.g. the transceivers which are supposedly not CAA approved. I think one could put together a sufficiently different product range.
Anyway, any physical shop must have an online presence these days. The question then becomes... if you are running an online warehouse, can it run from the back of the shop, with the shop front serving customers who want to visit physically? It must depend on how much of a premium you are paying for the rent+rates, versus a warehouse somewhere.
Harry M seems to be doing well but he is a Garmin dealer too I think, and sells a range of certified avionics too, with fat margins, and these go to installers only.
I've started two businesses in two recessions and it is the right time to do it. You get good deals from the suppliers and the landlord.