Welcome to the guild! like Angel said, age doesn't matter here. Well... you'd have to be at least 13 to be on gaia... though I've met some as young as eight on here. and some as old as 63. and anything inbetween 3nodding
quoting is fine, but try not to requote quote after quote. for instance.
you quote someone, then respond. they requote you and respond. then you reqoute their requote etc., etc.... it's and endless cycle and takes up a lot of space.
for selling things you can sell most items in the shops. for bugs/flowers/trash/ink/credits you can either use them to get new items from some of the shops (Barton flowershoppe, Skin tyte, H & R Wesley, Durem depot, and Gambino Hatrack) by clicking the exchange items dialog box below the shop keeper.
Otherwise you can trade those items for gold by using the exchange forum, or going to the gaia marketplace.
if you have any questions about gaia feel free to ask here, or PM me personally. I'd be happy to point you in the right direction, or answer it myself.
Hope you like Gaia!