I’m trying to work out the best strategy to get enough coins to buy the very expensive greenhouse...

There is no “strategy”. Either you keep playing the same scenes over and over again for coins, and not buying anything, therefore not being able to advance to the next level, or you just need to purchase the amount of coins that you need. They might have a special sale on coins, or coins as part of a bundle set that you can buy, thus making the coin purchase a better deal. They want you to buy coins, that’s why the cool stuff is so expensive! LOL!

You need to be careful when you sell; sell too much and you'll get into too much flower debt. Be aware you'll only get half the coins back that you paid for it. It's better to store and just replace it with an item with high flower value. But if you are too impatient, like me, this is how I sell. I set my sights on what I want to buy (usually the item with the most possible flowers), say it costs 10,000 coins. I replay until I get at least half those coins, better to be a couple thousand more, 7000 in this example Then I sell items of low-flower value until I meet my goal of 10,000. Then you won't have sold off more flowers than you will get, and you will free up some island space.