I don't know where you live and how densely populated your area is, but in general, online retailers are massively taking over and specialty B&M shops can no longer survive. Shops are merely a showroom for online retailers like Amazon.
Additionally, I suspect that no more than 15-20% of the Tennis playing population frequently updates gear and apparel, and restrings racquets.
That seems to be a really small sliver of potential customers.
Not to poo-poo your idea, but there seems to be easier ways to make money if you want to run a shop. Like cupcakes.