I'll avoid complaining about GW prices in this response, as it is not tolerated here, and also, we can't change them, so there is no point. I'll give my opinions on what I think is the best purchasing strategy though.
If you have the money, then most definitely I would say buy from GW. It means that you will be giving them direct sales in the lotr/hobbit range, and thus they will hopefully give it more support. Also, buying direct from them means if any products are defective (such as first generation finecast), it is easy to sort out.
If you don't want to do that, ebay is your best bet I think for metal models (and basically your only bet for oop models, and there is unfortunately a lot of them). However, even though you can get troops from ebay sometimes with good auctions, I personally prefer to get them from GW BNIB, as I like assembling armies myself prior to painting (most troops on ebay are already assembled, but if that doesnt bother you, fire away).
One thing I've noticed though, is that boxes such as the Winged Nazgul don't appear on ebay too often, and even if they do, they actually tend to cost more than buying direct from GW, so I tend to get all boxsets like that direct.
Of course, it also depends on what you want to buy. I am a collector rather than a gamer, so armies don't come into it really for me, instead I am trying to acquire one model of each pose. In that regard, I prioritise the lesser-bought models that GW currently stock, such as Gandalf and Cart, as these are most likely to go oop soon, and since they aren't too popular, won't really appear on ebay.
So, basically, my purchasing strategy is direct from GW first for all the stuff they currently stock, prioritising the not-so-popular models first, then once I have them bought up, its off to ebay to try pick up the older metals.
Hope some of that helps