Why would you buy them without knowing? The one in the picture looks a bit old and dirty. Are there really "fake" transistors?
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Unfortunately, fake transistors do happen, especially for good or expensive transistors. Often the lettering is a give-away - so find an example which is known to be legitimate and compare with what you have. Next, try running it at near highest rated power and see how it performs, and compare it to specs.
This example looks to me like it might have been used by the time of the photograph.
Just my $0.02 worth - not an expert opinion by any means. Good luck - and your uncertainty is exactly why I buy from a known source, so I don't have to wonder.
Sometimes. As mentioned, the lettering is sometimes different. Also, pictures of the parts may indicate prior use or even hard use. Then there is cost - if it looks like an amazing bargain - maybe it isn’t such a bargain. You could ask the seller where the parts came from, and even ask him to prove it with a bill of sale.
I value my time enough to buy parts from a recognized and respected dealer, or even better, from the distributor. But I understand that not everyone can be so picky.
Whatever your situation, good luck!