Hi! I'm pretty sure you're allowed to make a trade list even if you only have non-MLP stuff to trade. Someone else maybe can confirm that for me, but I've seen it done before. Also, I'm pretty sure you don't need to have a certain amount of posts to have a trade lists/take part in trades. Definitely helps to accumulate feedback, though people will understand that you're just starting out
If have feedback on Ebay or Etsy, mention that and provide a link.
As far as posting images, I personally use Imgur. Once you sign up (it's free), you say "Upload Images" (it's a bright blue button) and upload the images to your Imgur account. Once they're uploaded, you'll see them when you look at your photos; click on the photo you want and copy the BBC code (you'll see it on the right side of the window.) Paste the BBC code into your trade list (if that's where you want the photo) and you're done!
(You can also use Photobucket, like many Arena members do; here's a thread on that:
http://mlparena.com/index.php/topic,341766.0.html)