How do you buy / sell such heavy things like speakers over the web?

Local sale. Unless the (heavy) component is priced well below the market value, the shipping could cost more than what its worth to the potential buyer.   

+1 for Craters and Freighters, used them twice, a bit spendy but worth it.
+1 for Echo Audio too, especially since the new store has parking!
I also would be happy to help you get the Krell packed and up the stairs if you need it or get speakers un-crated and moved downstairs & setup.

For big speakers you can get them picked up and shipped by a company like Craters and Freighters. I have also used Uship a number of times for very large speakers across country at very good rates. Usually with Uship the company that ships the speaker blanket wraps it like regular furniture. If you specify they will pick it up wherever you have them and get them on their way. 

We're in the same boat. My Talon Khorus are also very old and while they were highly respected performers in their day and still damn fine right now I just don't see anyone paying to truck them cross country. I'll be using a combination of AudioMart and CraigsList. Might list here as well but wherever they list I just don't see big speakers being shipped.

Big amp, maybe.

The way you get it in the box, btw, first place it on the floor upside down. Support it, if need be, in a way that allows you to wrap it and put whatever packaging insulation is used around it. Then slide the box down over it, tip the whole thing up and over, finish off whatever goes on top, seal it up. Works like a charm. 

carried downstairs to the music room, out of their crates and tipped over to get the spikes on (and then stood back up)?

For this it helps to have a kind and helpful neighbor.

Echo Audio

On your place in Portland there are plenty of places and plenty of craigslist sellers you can get from instead of hassling with shipping heavy items. That's what I would do.