[Lucid:]

Sell back is 50%, correct.

The reason that the shop doesn't preview items on top of your current avatar is because of the layering system. The only thing it could really do is stick the item on the very top layer, and depending on what you are currently wearing, it might look pretty dumb that way; for example if you were previewing tall boots over a long skirt you were wearing, the boots would show on top and look really silly. By previewing it on a naked avi, you can see what the item looks like all on its own and then judge whether you think you like it.

I use a ton of trial and error in my layering. I typically pick a few items that I think will layer well, and then rearrange them, pick a few more, rearrange again, delete some, add some more, etc. Sometimes I set out to make an avi with a certain item, and by the end I'm not even wearing it anymore!

One tip I like to use, with small commons like socks, hair, or accessories I often equip 2-4 colors of the same item and switch around which one is on top, so I can quickly see which color of the item I want to use.