Concerning the coin itself, there are aspects of it that give me pause.
Concerning the seller, there are aspects there that give me pause too. They are not listed on the FORVM Notorius Fake Sellers List, which is good - they're not a known and unrepentant seller of fakes. They do have a lot of ancient coins for sale right now, and most of them look OK to me. However, their attribution skills aren't great - for example,
this coin is described as "Constantine II or III", but the legend on the coin is DN THEODOSIVS PF AVG, so it's an emperor named Theodosius (probably Theodosius I), not Constantine. So I don't really have full confidence in their ability to discern a high-quality fake - though I do note a heartening lack of "tourist fakes" being offered as genuine. So I'd judge the seller as "trying hard, but not necessarily perfect". They would probably be honourable to refund any coin considered suspect.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis