I've had a quick check on eBay and some market research tools, and found that there is a large number of monthly searches for these products. In other words, it's quite a big niche market. The sell-through rate on eBay is good but I would recommend that you check out your eBay competitors and the average selling price of these items yourself. This will help you know how much to pay wholesale in order to make a profit. You can do a quick search on eBay for MMA/UFC shirts for the active auctions or do a completed listings search to find out the sell-through rate of these shirts in the past 30 days.
To do a completed listings search, simply click the Advanced Search button next to the main eBay search box. Enter your search keywords as usual, and then check the Completed Listings box. Hit search and you'll see historical data for the previous 30 days. Successful items are in green; unsuccessful items are in red.
Hope that helps! I wish you all the best!