Playstation 4's related medias are, contrary to popular belief, region locked.
First of all, the guy is offering intercontinental mail while selling a product without speaking about customs because he's probably thinking of avoiding them, and that would be illegal by itself (but everyone does it tbh), but the fact that he is selling undiscriminately to every region either means:
1. He has no idea that PS4's games/ DLC/ Online will be region-locked.
2. If, for some reason your PS4 has a problem you have to send them back to Singapore... because you do know that Sony's Support is region-locked too, right?
And the price is fishy enough to raise suspicion. Let's consider the possibility that it's illegal: if he got the PS4 by selling smoke is a thing, if he cards them, they'll probably be blacklisted so, we go from "almost certain game problems, almost no support and almost no online" to "no support and no online".
If you are willing to pay for a PS4 that probably can't be played online and with a virtually voided guarantee, though...
tl;dr: You should probably buy this ONLY if you are from Asia. Otherwise, do yourself a savor and wait until the price drops if you don't wanna pay 399 bucks for a new PS4 (a working one).
I also think he is carding the PS4!
No I am not dambros89!!