One issue that really bothers me is that many teapots appear to have a coating of slip on their exterior. For those of you who don't have any experience with pottery/ceramics, slip is a solution of liquid clay that is applied to the surface of leatherhard (almost dry) clay before the initial firing. Based on my experiences with slip, the following pots appear to have a coating of slip.
A pot from "5000friend" on ebay, notice the difference in clay on the outside, and the inside of the spout.

Can anyone recommend a source for high quality yixings under $50
(I said that the pots were "glazed" in the title because I didn't think that many people would be familiar with slip)