In principle I think it would be a good idea, but it will never fly (sorry), since it would not only cut into the airlines' baseline, but totally sabotage the concept of yield management (or price discrimantion, if you prefer), which is totally essential to the business model of an airline (or hotel, or car rental company).

(https://en.wikipedia.org/wiki/Yield_management)

While this may seem frustrating I can - to some degree - understand their position, since there would be a business model in that.

A cabin is divided into a number of classes. That's not just first -, business - and economy -, but economy class as such is divided into a number of sub classes, which are sold under different conditions for different fares.

For example: Swiss offers the following fare classes in economy (I assume this goes for the whole Lufthansa group, but didn't check). From most expensive to cheapest:

  B, Y
  G, H, M, U
  Q, S, V, W
  E, K, L, T

Now they have a certain amount of seats available in, say, fare class K. Once they are sold out you can only book seats in classes S, H or even Y. It fluctuates, though. They may add more seats to cheaper classes if the flight is in danger of not selling out.

Let's assume that I buy K class tickets for a flight, from which I assume that it sells out well in advance. Two weeks before the fllight is scheduled and only (much) more expensive tickets in B and M class are available I offer my tickets for sale and share the savings with the buyer, who, in theory would have been willing to pay a much higher price.

So in essence it's the ability of the airlines to price discriminate, which would be endangered if tickets are resellable.

You can argue that this is not right and unfair. Then again it's price discrimination, which enables very cheap flights. Provided you book in a timely manner and accept limited or no flexibility.