Hi:
I have used SELLER SOURCEBOOK for a long time now and am very happy with it. It is a pay site so I fork over a small fee every month for it but I feel its worth it. There are lots of free template sites out there for you to try as well.
Does it increase sales? To a certain extent it does. Personally I like to buy from someone who has a professional looking ad. I want to be able to find the description, terms, shipping, etc. easily. If it takes a long time for me to navigate the ad I simply move on. Please note that I said "professional" looking. I don't find that my sales are higher for items that have pretty flowers or elaborate designs. A solid color border and neat organization works just as well.
Ohhhh....and while I am on this topic....I NEVER buy from people that have extremely busy ads. I find music and flashing mouse trailers completely annoying.
You just have to strike a balance and make sure everything important is right where your customer can find it.
Joy