Pretty much this ^
Buildings are absolutely useless at earning coins (the best building is far worse than the lowest coin producing dino).
Sell any and all decorations maybe in the last few minutes of the trade being available.
Don’t use or buy the lower level decorations. I have a guide to the best decorations available to use.
If you can’t get the 13 million then start investing in more (worthwhile) decorations and park layouts that increase coin production.