Included! Forgot to put this one in the first time :O it's a very good method!

I often sell my spare evolutionary items and, of course, treasures, directly from my inventory, which helps a lot to get quick credits

An idea I have for earning zophan canisters is to sell boxboxes. It isn't very challenging to get them, since it only requires doing scour missions to get normal boxes. The "average price" I see for them is 100 zc, though many people price their boxboxes to 110 or more, it's really the choice of the person who's hunting the boxes. With all of the scour missions being done, people can also get credits from selling evolution items and berries that they get in scour missions.

Gonna post both of the above suggestions!

Ill often sell my rewards i get by ranking up, since they have a pretty high market value. I look in trade shops too to see if I can find people selling things below market value, and then sell them for a higher price. Some people get iffy about you doing this though so make sure to read their rules.

Added, thank you!

Interacting also gives you Interaction points, which, if you have enough, lets you buy the Albino Radar, which sells for 125,000 credits from your inventory, meaning that interacting does give you a nice boost to get credits.

I think you mean the Albino Radar. You can't buy the pokeradar with IPs.

Right, I always get the two mixed up due to their similar names. I'll edit my post for easier copy pasting if it gets added to the guide.

For earning credits,I suggest going on scour missions.Sometimes,when you go on scour missions,your Pokemon find credits sometimes.There are also a few items you could sell to get some credits,for example Nuggets,Pearls,Big Pearls,etc.