Just because something has been removed from it's packaging, it doesn't magically lose 30% of it's value. Sure, it may not be brand new and sealed but who really cares if something has been opened and test fitted and then put back in the packaging. As long as someone hasn't fucked it up with scratches and stuff from the fitting, so what? Is it any different from a retailer, in this case Airsoft Global stating this?
"Whenever we test any guns, scratch can't be avoided.
Therefore, we are hoping that you may consider carefully before you purchase it!
But we can guarantee that all items are brand new."
If someone has gone to the trouble of going through the waiting time of importing something, paying customs fees etc, I see no reason why you wouldn't pay a bit less than the "brand new" price to save that time and end with something in the same state as it will be when you open the packet.
The whole paying full price for customer support and warranty is also kind of overrated. Sure, for some stuff you absolutely want that in case it's properly borked but I'd guess lots of things can easily be sorted for lower that the cost of return postage to a retailer. You just have to evaluate the risk on an item by item basis. You're pretty safe with accessories and similar, especially ones that don't involve any electronics.
I think at the end of the day, just be honest and make sure stuff is described accurately. Now if people are trying to pass stuff like a RIF off as "only skirmished once, as new", and trying to get retail price, now that's a no no.