Should the seller discount what they are selling a certain percentage since they aren't paying taxes?
I'm not sure how much of a discount, but maybe 5%?
I'm looking at this as a buyer that would pay cash for Bitcoin.
Buyers and sellers have to pay tax according to tax laws at their nations. They are responsible to file their tax reports too.
Why do you care whether your trade partner pay taxes for his/her trades? It is not what you should care of.
When you trade Bitcoin with cash as a payment method, you will also need check your trade partner's reputation, trust, and trade history. It can only be checked if you do the trade on a third-party service and if that platform has those metrics for users.
Some trade partners might have money laundering activities and they are usually new accounts and will give you very good discount rates. Pay attention on discount rates, if it is too good and account age is too young and with very little past trades, please consider a few times before you trade.
Police or government can ask you to refund those stole money to the real victims and you might not get your bitcoin back.