Bobl, I have worked in a company that used to pull this trick on our clients, lets put it down to experience. Build up big sales but on credit, then come back to the supplier to negotiate. It works both ways sometimes as the discount may be passed on to the end user, or may not.
I am not the world’s best businessman by any means, but I suggest you look at your operation as the 20-40% profit margin you are talking about are too small to cover your credit risks. Think about trying to include some upscale stuff, up and coming or limited availability stuff in your catalogue. Try and lock down on something very nice even if it is only 20% of your sales. When they try to play the game with you they will risk future revenue and pissing you off. I have worked as both a supplier and distributor and I think that is one way to handle this situation for the smaller fish.
Also have a bottom line and don’t extend credit beyond a certain amount or certain time period, even if the deal is supposedly lucrative, if you risk losing everything on one deal. Taiwan can be unstable, some people have an exit plan where they screw all their supplier for credit (after two or three years timely payments) and then f%^k off to China with all their suppliers owed monies.