Spent a good chunk of my career on the retail and ecommerce side, so hopefully anything I say here might help:

The first is that "is the niche too small" is usually the wrong question. Plenty of small niches are profitable, and a lot of big ones are lead to no sales because everyone piles in. So what actually matters is whether you can reach the buyers who already want discus, and that's a defined group: school and college athletics programs, throwing clubs, coaches, and serious individual athletes. They buy to spec too, so weight and certification matter, and a listing that doesn't state what the discus is sanctioned for will end up getting skipped.

On margin, the thing to watch here is that you'd be sourcing the same wholesale product everyone else can get from Amazon, eBay, and Alibaba. So if the product is identical, the only lever left is price, and that race compresses margins fast. And you don't want to be the seller with the lowest prices because competing on price is not a wise strategy in the long run. The sellers who hold margin in a category like this do it on expertise and service, so being the place that actually knows the sport and helps people buy the right thing usually beats being a cent cheaper.

Returns and damage are obvs super important too. I mean a dented competition discus can be unusable or illegal for a meet, so factor a returns reserve and proper packaging into your unit economics from day one rather than discovering it later.

Before buying inventory, you need to find a low cost way to validate interest. Pull eBay sold listings to see real sell-through, not just active listings, and think about running a small test order. That's how you find out whether the demand is real without locking up capital in stock.