Tools and supplies for my bladesmithing hobby. The supply sites that do exists have really terrible web presences and they still do much of their business with paper catalogs.
Most of the stores don't have items categorized more than one level deep nor by the properties of the item. There are all sorts of options for something as simple as an anvil or a pair of tongs.
God help you if you need handle hardware. You have to go through 40 pages of little, crappy pictures of items to find what you're looking for.
When you go to checkout, you're lucky if you can complete your order without reloading your cart a half-dozen times.
Then there's the whole quality thing. There are vanishingly few reviews on these sites. If you could aggregate reviews on similar items, it would be great.
But a problem with aggregating reviews is that a certain part might be imported from China on one site and the same part might be laser cut in house on another site. There's no real way to tell the difference without having the two items in hand.
Edit to add some links to examples:
http://www.knifemaking.com/Default.asp
http://www.piehtoolco.com/
http://www.usaknifemaker.com/