In the long ago good Olde Days of Games Workshop (read early to mid 1990's), I was collecting many Space Marine and Titan Legion armies - 6mm scale approximately. They did discounts in those days (
) and I placed a fair-sized order for Reaver titans and some other stuff. The entire order went missing in the post. I contacted GW. They despatched a complete replacement order and I had it within a couple of days. Now THAT was great service.
I have never bought off eBay and got burned years ago when BattleGamesByMail went under. Nowadays I tend to buy only off long established internet stores that also have physical shops where you can see lots of activity. I have had no problems for a long time and hopefully it will stay that way.
Good luck with GW. They should at least send you the missing bits.