I’ll get the ball rolling:
1. Skin. Some skins are just more valuable than others based on aesthetics and rarity, as there is more demand/less supply. For example, eternal/ghostly and zodiac weapons only drop in certain places, making them more valuable as good ones drop less often, and the skins look nice. That being said a nice thing about gw is that valuable drops can come from almost anywhere - a common skin with some of the below features can have good value. Similarly, rare skins drop in unlikely places (non insc paper fans and diamond aegis drop in kaineng city, far from an elite area) so I wouldn’t get hung up on merching things based on where they drop.
2. Insc vs non-insc. Items that drop in prophecies and factions (there are exceptions) have fixed inherent mods I.e. an inscription cannot be added. Good inherents have lots more value because you need a skin + inherent to drop. With NF and EOTN items, you can add your 15^50 later on, so value is more dependent on skin and req only. As a result of this good insc weapons are more easy to acquire, and thus have less value. This effect is exacerbated when inherent mod combinations are very rare to drop e.g 20/20 matching mods on staves, wands and foci. Perfect mods are also worth A LOT more than even near perfect ones.
3. Requirement. Max weapons drop r9-13 in most cases. R9 is much more valuable simply because of aesthetics again, but it also allows you to run lower attributes and get maximum value out of offhand items. R8 gold max swords/shields/foci do occasionally drop but very rarely, and do have good value in most cases, with the exception being non-insc single mod shields/foci and bad inherent swords. Purple r8 weapons can also have value if insc or a nice inherent combo.
4. Inherent mods. Non-insc 15^50 and +5e carry the most value. Good skins with 15stance or 15ench can have modest value also. 15 -1hp and 15 -1e drop more rarely and also have very good to modest value depending on skin and a low req. Other inherents have value once you go below r9.
5. Prenerf items. Items that do not drop any more are valuable as they can no longer be acquired without trading. This includes r8 items not detailed above and all r7 items. This is probably a whole topic on its own.
I’ll stop there, other than to say that these effects stack multiplicatively. A rare skin with good inherents and a low req is going to be worth far, far, far more than a common skin with an okay inherent and a higher req, with the difference being in the 10s to 100s of ectos.