Don't make stuff up or pretend that you know more than you do! Do your homework and know your stuff. Also, if I see one more tacky 80's print listed as "art deco", I may scream.
First of all, you deserve to make what your product is worth. By undercutting your competitors, you're undercutting yourself. If 10 Etsy sellers are selling owl hats for $20 each, they've made that the industry standard. If some fool comes along and sells theirs for less, they're not making what they should and they're making shoppers think the other hats are overpriced. That hurts the whole market and their own profit margin. It's really hard to keep a crochet business going if you don't have money for new yarn!
This goes for vintage items too! If everyone undercuts everybody else, no one will make money! If you're not sure what to charge, research it!
The best help you have is right at your fingertips! The Etsy community is the best thing about opening your shop on Etsy instead of somewhere else! If you have a question, there are forums you can read that will probably answer it. You can even start a new thread with your question to get answers. Most Etsy sellers are more than happy to help new sellers. You may think of these people as competitors, but you should be thinking of them as colleagues.
You'll be a more powerful seller if you work with your new colleagues. Joining Etsy teams is a great way to get your items promoted by others. All you have to do is return the favor! It may seem counterproductive to promote for others. However, if you're tweeting or pinning or facebooking their items (and your own) and they're doing the same, everyone gets more exposure. Take full advantage of your new friends! Branch out and network. You're business will grow so much faster!
That's all I have for now! I spent the weekend moving and I won't have internet in my new place until sometime Wednesday. Hang tight and I'll see you soon!