My first experience of this when last year when Bob told me that he bought some Space Wolf bits in order to get a particular 'thing' for one of his units... and he then sold the rest of the bits on for the same price that he bought them. Having bought my Cypher bits from what basically seems to be 1 seller selling individual parts from the Dark Angels Chapter Upgrade sprue I am wondering if there is a market to make a reasonable return from buying these kinds of kit from GW and then re-selling as individual parts.
This spue costs £20 from GW, which I think is pretty good value considering the variety of bits you get. However if you consider that the torso's, robes, heads, banners, heads could be sold for £1 each then its possible to make a profit... if you can be arsed to list each item and then post off the orders when they are bought.
Then we have the second market for units/entire armies. With the general price of GW units being so high eBay seems to becoming a massive market for GW products. Its great to browse and see 1st/2nd edition models and reminisce back to those glorious days when Blood Bowl was the new kid on the block and each GW store had more than 1 member of staff!
I don't really any profound message or conclusion about the GW eBay market. I just love its vibrancy and hope to get more involved in it myself. I would love to hear anyone's experiences of using Ebay for buying/selling GW stuff.