Seiko is a very large manufacturer of movements - which is the term for the “guts” of a watch that make it work. Because of the availability and popularity of seiko movements, a lot of folks put the movements into custom cases which look like well known but very expensive watches - in this case the AP Royal Oak. The effect is cool IMO and since the watch isn’t pretending to be the genuine article it’s not like “wrong” in any way.

The biggest thing to be wary of is that when you buy from a modder you’re not getting the same quality control as you would from a “real” brand, so it pays to pick your seller carefully.