As a new player, is there a way to know which merchant offers are good value?
Or are they all good value?
It’s a shame that the chat feature is not available on the Merchant screen. I don’t know which later game consumables to grab as soon as I see them offered, and which I can ignore until I actually need them.
Imperial Deeds and Celestial Traitstones are pretty good buys with gems, and everything except ingots is probably worth their gold price, especially the pet food. Most other stuff is probably not worth the gems, except maybe dragonite, but if you’re a new player you don’t really need to worry about that for now.
On the gem side:
Regular deeds are an important pick, and one, that is a lot cheaper than elsewhere. And you will need LOADS of those in endgame. Imperial deeds can be useful, but are not as urgent at your projected progress speed.
Underworld stuff (chaos shards, high level treasures purple upwards) should at least be something you consider, as you will have to catch up a lot there.
Diamonds are not strictly necessary and can be skipped. You get a much better deal out of the sunday dungeon offer.
Tokens and vault keys are quite useful, but also expensive. Decide for yourself.
Dragonite is skippable. There are only limited uses for it (although very good ones), and even without spending gems, you’ll have all of them fulfilled within two years.
Celestial traitstones are not urgently needed. If your guild is halfway decent, you’ll be swimming in orbs of wisdom eventually and can do the highest trait upgrades this way.
Not sure about runes…
Gold side:
Get writs.
Pet food is a nice catch. If there’s nothing great on the gem side, grab it.
Keys and shards are always good to have.
Everything else is not noteworthy. Souls and ingots are throwaway trash even at early midgame already, low rarity treasures (grey, green, dark blue) will equally pile up due to their low usefulness rather soon.
gotta remember you need celestials for crafting troops/weapons in the soulforge as well, as a late-game player myself, ive only JUST hit the point where there is nothing left i really need celestials for. But yea, regular deeds can also be worth it.
My advice is from a more “distant view” of things: Know what your budget is, and know what your needs are. Be very selective when it comes to spending Gems, especially when weighed against your Gem income and the other stuff you need to spend it upon; you can be less selective (but not completely profligate) when spending Gold on stuff given how much more easily that can be farmed.
Colored Deeds are a terrific value when you can afford them. They’re also an item of considerable need once you progress far enough; this combined with how difficult it can be to find and accumulate them if you don’t spend real money makes them a very high priority for a lot of players. That being said, Writs may be an even better offer when offset against Deeds in a side-by-side comparison, at least if you already have the other necessary resources to convert them because Writs cost Gold while Deeds cost Gems.
Anything you have in sufficient supply can be safely ignored. Anything you can semi-reliably farm on its own can be safely ignored – even if I didn’t have max Renown, I’d probably put Shards and Treasure Troops in this category because it’s quick and easy enough to run three quick Daily Delves at Level 50 for loots and searching for Merchants.
Even so, a lot of things that only cost Gold are worth purchasing if the Gem option is lackluster because it’s probably easier to farm Gold than it is that other stuff. (For example, I’ll buy Keys of all sorts and even Souls if the Gem offer doesn’t do anything for me.)
But your mileage (and needs) may vary.
Thanks for much @Schaafy, @Jerry_the_skunk, and @Kezef for the advice. 
It was very helpful to read your perspectives.