only trade legendary golds if you dont need them. look at the stats and see if they will be useful down the road(you also might need them to create a hybrid). generally, the gold creatures take 7 days to hatch. so if they need to get to level 30 or 40 to be useful, then they are probably not worth hatching. most vips are unbeatable at level 30. very winnable in pvp at level 20.
I assume your still doing the story fetch quests. plan ahead for those. when you have to feed 40 dinos or get to xx level 40 dinos or have xx of say amphibians, it can be a hassle if you dont plan ahead.
hammond and fossil are the top decorations not sure when they unlock, but they are a great way to store coins if you need them to make a trade harbor trade. yes you loose 1/2 purchase price selling them, but when you can get say 40,000,000 in food for 80,000,000 in coins, sell the extras!
early in the game, dna and food is not that big of an issue, but when your trying to ge the higher end dinos that cost up to 70,000 dna, your going to wish you did not waste dna. and when you trying to level up those big beasts that take tons of food, you will be trying to keep you food level up.
as a rule, always take dino cash in trade harbor. Never spend it in harbor unless you see a good deal on a mod you might need for a daily challenge prize (regenerate, toxin, inspire, predation and terrify are all great mods I try to keep 4 or more in stock). dino cash > loyalty points > dinos > food > coins.
I never fight in the pvp modded arenas. You get better prizes in the regular battles. It might be worthwhile to gather sdna, but I still have enough sdna to hatch 14 indoraptors. So its not a high need. doing the daily events slowly racks up the sdna.