It is not wrong what you are doing. The problems arise when things don't show up, or they receive faulty goods. I sell books and I am only happy when I have the book in my hands, and checked it over personally, before I stick it on e-bay..
If it gets sold then I get a proof of postage receipt which I keep. Even after all of this, you can still get problems.
Whoever you supplier is, can you talk to him about a discount for a bulk buy? This again would save on postage to yourself. Its a matter of choice. Hope this helps.