Q. Is it possible to buy a "anytime" ticket between two destinations? I would like to buy a ticket between PDX and LAX that I can use anytime I want...but none of the websites seem to sell anything like this.
A. It is, sort of.
What you'd do is buy a fare on Southwest Airlines. If you decide not to use it on the original dates booked, you can cancel it without penalty. Then Southwest will give you a flight credit good for a year from the original date booked that you can apply to the route. If the fare has gone up or down from your purchased fare, you'll either pay more or keep the fare difference in your account.
The other way to go about this, of course, is to buy a full fare, fully refundable ticket, and those don't come cheap.