You mean not finding online something that one can buy in a brick-and-mortar store? Never. The only time I can't find something online is if I am mistaken about its existence, or it is a discontinued line, or is a rarity.
Of course, there are many, many things that are uneconomical for me to buy online because of the shipping methods offered.
I was intentionally vague; I had mostly your first point in mind but was also curious to hear of things that are uneconomical to buy/sell online. I asked the question more out of idle curiosity than anything else, maybe something here sparks conversation or inspires someone :)
AFAIK, real estate falls into that category. (To be fair, I was researching, not actually attempting to buy online)
Certain categories of food would also fall into that category, especially if you're not in a major city (e.g. fresh seafood, delivery for some types of cuisines).
If you're looking to find some blatantly obvious material good which would be easy to sell online, in the hopes of making a quick profitable business, I'd reckon statistics are against you.
No, I'm not looking into getting into ecommerce as I have neither the time (I start my masters program in two weeks) nor the money (I start my masters program in two weeks). Just thought it might make interesting conversation.
I think I've read about some uber-hip fashion designers who have no Web sites and the only way to buy their wares is to line up at 5AM at their store (which has no sign, natch) located in a run-down side street in Brooklyn. Or maybe that was a William Gibson novel...
Of course, there are many, many things that are uneconomical for me to buy online because of the shipping methods offered.