My steadiest source of silver is quests and exploration. Quests are more often than not a luck thing, because sometimes I can do all of the quests available and sometimes I can only do 1 or 2. Really this depends on my pets' genes and what the quests want from me. I don't have every pet/gene combo, and some I haven't yet gotten from buying pets or breeding them.
When I have pets available for sale, I get a little bit of silver out of them, depending on how much they sell for and how quickly they sell. To sell a pet requires Vermillion and Silver, as I tend not to put unresearched pets up for sale.
When I breed and sell a deviant pet (which is very rare), it usually sells for a large price. I was able to get quite a bit of money from selling two Roc deviants, which helped jump me to 1 million silver.
Selling items is kind of iffy, because most often the items I can sell are not worth much, have no listed value, or no one wants them. I end up with a lot of duplicate schemes from exploring and try to sell them for 5 silver apiece because I know people won't pay more.
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