Welcome to res, Exo, so glad you're enjoying your time on here so far! :D
For trading/selling it's a good idea to look at the item in the item database before selling (especially when you start accumulating rarer items ;o) because 'when an items been sold' and 'for how much tu' can be good clues for selling at a reasonable (not too high or low) price!
Selling eggs is a great way to make tu as a new user.. some eggs like ardur/tesuri/intes can be sold from 200k-900k per egg because of how rare they are.. people want quick incubated eggs, and sometimes it takes a while to find them!
If you're into selling items, there are limited items that come around every now and then, and those are REALLY good to sell. Especially if their food or books because 1. Books can break randomly and food are single use, so the rarity and prices goes up and 2. They can be used for a pets intellect/nutrition respectively for neato prizes (if you go to your profile and click the blue links next to the corresponding stat of the pet selected you can check it out!) Also, limited HA (human avatar) items are good to sell.
If you have any questions about anything feel free to hmu in chat or send an rmail! :)