With Hikvision and Dahua, your support and warranty pretty much come whomever sold you the equipment. Neither Hik nor Dahua typically deal directly with end-users.
Something to be aware of when buying from unknown vendors on eBay, Amazon, etc is that both Hik and Dahua make models only meant for use within China, so the firmware on them only supports Chinese for languages. Some vendors put on a hacked version of the firmware that supports other languages then sell these cameras outside of China usually for cheaper than the true international version of the same model. You typically can’t upgrade the hacked firmware on these models so the usual advice is to stay away from those models.
If you have any brand flexibility, check out the Dahua subforum on here. Dahua’s stuff is comparable to Hik (IMO) and the forum here has a great vendor (Andy, @EMPIRETECANDY) that makes ordering them easy. With a ton of forum members here using Dahua, there’s a ton of knowledge in that sub-forum, along with a bunch of review threads.