Soooo.... i have several hatch lings I would like to sell and i want to post it on a forum but i have no idea how!

Start a thread in the pet auction forum by doing exactly the same thing then you did here.

If you want to post a link to your hatchery / pets there is a button called URL that allow you to do just that and for the picture it's the one called IMG.
To usa any of those you just nead to put the image url (for IMG)/ link of the page ( for URL) between the two part that will appear lile this :

[ url] *link here *[ /url]
[img ] *picture url here *[/ img]

( keep in mind that the space in the exemple is there to allow you to see the texte and is ont part of the real code )

i hope it help ^-^