But what if relays issue their own specialized ecash stamps? Users would purchase these ecash stamps from the relay using Lightning Network sats. 

Isn't this similar to an App coin? It's inefficient for every app to issue its own specialized coin; we only need a universal currency: sats. 

Relays should accept ecash sats issued by as many mints as possible and blacklist bad mints. nostr:note1re4sur0xjd8wxq0wqk8y5qjpp085kyqu7f5fe6zuz3p0e5kvrgfsxz4892 

 Could non redeemable stamps still be exchanged back to sats on a secondary market, non atomically and with a discount? 

 Yes, as long as there are buyers. However, using Ecash sats directly would be more efficient. 

 Perhaps ecash tickets are also more suitable than ecash sats for certain scenarios.