I always welcome competition, many here have already stated the obvious where the larger retailers have closed the gaps as best possible. There are also more than the three mentioned online computer stores. Wootware, Rebeltech and Evetech pretty much have the market share secured, but this does not mean that you are hurdled to enter the market.
You will need tools, applicable to any vertical. These tools begin with a business plan, and importantly your go-to-market strategy, all which have to be within regulation, and non which may be thumb sucked. Then you need to reach your start-up goals… then the rollout and implementation, testing and ultimately by going public (or opening your doors to the market).
Penetrate a market where it is vulnerable, no market is completely monopolised… well, pending on regulations. It takes market research, time and more time. You need the capital.
There are many verticals to explore.