I have a theory that leaving feedback depends on the item. I'm more likely to get feedback for items that I have handmade (different ebay account). Overall I've noticed less feedback being left than in the past. I suspect the constant emails requesting feedback for non-ebay online sales and services has turned people off.

For example, every time I have a parcel delivered, Australia Post ask for feedback. 

Recently I bought shoes from a store online and they automatically 'subscribed' me to their constant sale emails and started begging for feedback.

If other buyers are similarly bombarded with what are really junk mail messages, no wonder they are less likely to leave feedback anywhere.

I suspect ebay sellers are more likely to leave feedback when buying as a courtesy to fellow sellers. Members who are only buyers are less likely to make the effort, unless they are in the increasingly small percentage of generously minded, unselfish people out there.