I only recently came back to this game after... uh, a couple of months. Didn't even remember in which state I left it.
Apparently, I had made 2,5 million from market sales. (Which I can vaguely remember.) I mostly sold stuff you can produce fast and easy or stuff with as few ingredients as possible. I invested my gems early on in tender sister decoration and market slots (got 9, at this point).

After coming back, I worked my way up from level 40 (the max back then, so I got bored) to level 50. Buying absolutely everything, always trying to upgrade my barn, buying all new buildings... I didn't even manage to 'get rid' of half a million. Still sitting comfortably above 2 million.

However, at this point, the train is basically the key to everything. I got impatient at some points and bought a couple of gems or took some offer or something, but you can totally get everything without ever paying a dime. Fulfilling the train orders and getting 3 keys together has a chance to give you something between 5 to 12 gems. If you already have all the adventures, even better. The other 'loot' is a growth buff for you fields, a barn upgrade (which is good in its own right) and a photo slot (which is basically free gems as well). So with the exception of the buff, all good loot.

At this point, I have all season 2's so far and my barn has a capacity of 1,300 and I'm currently working on collecting enough gems to bring all my shops up to 4 available crafting slots. I think I'm going to collect 350 gems extra, because after maxing out the recent season 2's and upgrading my buildings... I'm probably going into hibernation again. xD

Events where you have to fulfil orders are obviously a pain in the backside, as every production not focused on the train slows your progress in gaining more gems.

Yeah. That's my to cents, I guess. Hope it helps. Happy gaming!