You can keep all the buildings you want/like. You have lots of park room. I did all the story missions as fast as I could. Use this site to plan out for the big missions. Getting to say 40 amphibians can take time. also feeding 40 carnivores will teach you not to max them out as soon as you hatch them.
At your point in the game, most of your Dino’s are not going to be earning That much coin. Look for the ones that do earn and surround them with multipliers. If you watch the videos you can see ideas. Coin traps or setup rowes of dinos surrounded by 2 or 3 rowes of decorations. The thing to gather about decorations is look at range, size, and multiplier. So 3 hammonds take the same space as a fossil, but 3x7x muiltipler is 21X vs the fossil is only 10X.
As to how to get more items in your market, find some open space, buy say 10 fossils. Then click on them to pick it up and put it in your market.
When you have tons of coins, you can keep 200 fossils in your market if you like. But you can only sell 1 at a time. So keeping 3 or so fossils in your market is a good practice. Put the others in the park to help with multipliers. If you sell A few fossils, before you sell the last, pickup a few from the park. If you go to 0, you will loose those trade harbor deals. They don’t get changed to something else.