Hey!
That's a bit of a doozy, but I'll try to help you as concisely as possible!
For Bases:
This is the Combo base Wiki Page, almost any of the factors on here are a pretty good base to try! They are almost always popular depending on how pretty people find them, and whether or not they are natural/colorbomb breeders.

With Markings:
There are a lot of of different reasons they are popular, if you don't want to go too crazy into it then generally Tier 2, Tier 4, and Tier 5 markings are considered 'good'. To go into slightly more detail you can check here to see how rare a particular slot is.
That being said there are a lot of Tier 3, or applicator, markings that are popular as well. It's a pain to hear, but sometimes it's just what looks good that gets popular.

Manes:
There are some breed only manes, and breed only will always be popular because it's hard to get, but the Ochre Saltlick directly passes a mane shape and color so it's not as big a factor otherwise.

Mutations:
Mutations increase prices anywhere from 10%-100%. Mutations that are 'Easy' to get, like most Mane Mutes (and base Primals), are worth roughly 1-2GB with nothing else good about them, others vary. The best way to price them is to check the Trading Center for what they're currently going for. They get particularly popular when you can pass them down, which Kings can only pass: Dorsal Fur, Patches, Piebalds, Primal, Primal (Felis), Primal (Ferus), and Primal (Smilus).

Hopefully that helps! I'll elaborate on anything you need me to!

You have liked this post!