Hmm,in my opinion that should be electronics and gadgets because we can often see some deals online and also clothes I think is a big one.That should be the top selling products as far as I'm concerned.
I too believe that digital items are still the number one things being purchased, although that could change if we get some more adoption. 
DIgital items would be really on the top hence we are buying it using digital currency and we should think that digital things would be the trend but as we all know that people would surely made another way to sell of things and doesnt mind if its digital thing or not as long they could able to gain profit with that.
I agree with this digital items would really the trend right now on purchasing and using bitcoin on any transaction. Digital currency is being commonly used on these type of goods. As you mentioned shipment fee would really matter with this though thats why some potential buyers would really choose to buy on local rather than on online shops that accepts bitcoin because of shipment fee problems.