If they trademark the name (which they may be able to do, as they appear to be the first ones to use the name in a commerce and as an expensive facility they likely have a team of high-power lawyers on their side), they might be able to take the domain away from you by filing an
ICANN dispute.
There are three criteria that must be met in order for them to get the domain taken away from you.
First, it has to be their trademark. We don't know that's the case, but if it is (or if it
becomes the case), they've met item #1.
Second, they need to prove you have no rights to the name. Since it
is the name of the arena and it is neither your name nor a trademark or business name of yours (and opening a business using their name after the fact wouldn't count -- in fact, it might just make things worse), it's likely they'd prevail on item #2.
Third, they need to show you hold the domain in "bad faith." There are three possible ways they can show this. First, by showing you registered the domain simply in order to sell it to them at an inflated price. Second, by showing you registered the domain simply to prevent them from registering it. Third, by showing you registered the domain in order to put up a for-profit site and make money by leading people to believe you're somehow associated with the trademark holder.
They don't have to prove all three of these to show "bad faith." Any one will do.
So you'd have to be
very careful about what content you put on your site, or they could prevail on item #3. Especially since you've already said your goal is to make some money from this domain.
IMO, RayGoneFishing is totally correct. You really need to talk to a lawyer before you do anything further.
--Torka