For clothing/wearables, you simply just check the price of a similar item using an item database. I usuallly use Jellyneo or NC's
Try to match these as best you can:
> species
> rarity
> release date
Then guestimate a price you're willing to accept, then post it on the TP asking for offers, and hopefully you'll be able to get something around that price.
But be reminded that wearables deflate extremely quickly from the time they're released.