If you buy from a vendor over the Internet, make sure you use a credit card with a good dispute resolution policy. Most vendors will work with you on an issue, but at least one I've used (for RV parts) was totally schizoid. I finally just had to tell them they could either work with me on the issue or deal with American Express. They called my bluff and ended up on the loosing end. I even got my shipping charges refunded.
Other than that one vendor (which was an Adventure in itself) all the others listed on this thread have proven to be very good to deal with.