Acleacius, there are a variety of websites dedicated to showing what gear looks like. So when you find something tantalizing on the market, you can go on GW2db or GW2wiki and find examples of the items. Theres even a website (I found it by searching 'GW2 dye list) that has all the dyes and what they look like on various pieces of armor.
So yes, having a preview option on the tp would be nice, so would a way to put the preview character in an active stance so you can better see small weapons. But there are tuns of ways on the web to see every single piece of armor or weapon in the game.