I think you are confusing things a bit. There is a bunch of stuff you need to collect, which are quest items, i.e. no trade. Once you are done with all the prequests and you have the final dreadsteed quest you need to buy some stuff from the vendor next to the questgiver.

However, all this stuff is not single use. It can be used forever, but only by the char who bought it. That means, you need to buy the three final bits only with one char, that does the trick for y our group. You can also take this char along with anyone else who wants the mount to help them out. Just make sure you got plenty of shards, since you will consume a bunch in the ritual.