Why do people...

[UngaWunga]

...list airplanes for sale without listing a price?

Never understood that. I have no interest in calling you to find out if you're asking crazy money for your plane or not. List the damn price. Then, if it seems reasonable, I may contact you.

Maybe they're lonely and want to chat with people? Trying to collect phone numbers and email addresses?

 

[Jim Logajan]

...list airplanes for sale without listing a price?

Several possibilities:
(1) They are clueless nits.
(2) They haven't been able decide on an asking price.
(3) They know their asking price is above the going rate for comparable aircraft, but also know that they'll get about the same number of queries without an asking price as with one (i.e. very few.) The few prospective buyers who see something of interest in the aircraft that makes them call should be the few self-screened "highly qualified prospects".