You can sell your souvenirs in Albuquerque but it's usually only worth it if you have a high bonus multiplier at the time.
Otherwise keep the souvenirs because you can get super high end of ball bonuses with them.
Buying the pinball souvenirs (zinc, gold etc.) is also a good idea because with every extra ball you get an end of ball bonus off course.
For high end of ball bonuses you basically need to light as many cities and have as many souvenirs as you can. Top it off with a 6X multiplier and you're king. I've seen them as high as 6 billion.

Only souvenirs that have a city linked to them will give extra bonus if you have both the city and souvenir.
You may get money for the other ones if you sell them in Albuquerque but I'm not sure.

Here is the list that Pinballkim linked to:

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Just use the 'pause function'. Shoot the ball in the start city hole and before it ejects open the coin door.
This will disable power to the coil that shoots it out and you can leave the game while you stir.

If you are on your last ball in a tournament, sell, but if you've got some game left keep them for high bonuses later.