I agree, it all depends on what you are selling, what your price is, what your shipping and handling fee is, etc. It takes a lot of research, both on eBay for current offerings and completed sales, and off eBay for the same things. Depending on the item and whether I've been able to find such info on the 'net, I've even gone to my library for research.
One thing I've noticed is that if the item price is very low, but the shipping and handling fee is outrageously high in an effort to make the "real" selling price, most buyers click on past the listing and look elsewhere. I know I do!
If you've got a hot item, buyers will come quickly. If demand is low, then the wait can be awhile before someone comes along looking for it. I sell vintage and antique books (among other things) and often it takes months before a listing is even seen, let alone getting the item sold.