Convey information as clearly and concisely as possible on your artist website.
For example, when a piece is "Sold", just mark it "Sold." In a museum, it's nice to be "classy" and say that an unavailable piece is in a "Private Collection."
But online, I highly advocate using words like "Sold" and "Buy Now." People don't read online...they scan. So you have to spell things out as clearly and cleanly as possible.
Don't worry, people will still think you're "classy."
In fact, Alyson Stanfield recently had a post on her wonderful ArtBizBlog regarding this same subject and one of the commentors, Barbara J. Carter provided some feedback saying, "I used to use 'Private Collection' but the majority of my customers, and dare I say, fans, would ask me if a work was available or not no when I used that label."
So just keep it simple and use the word "Sold."
Software Craftsman and Art Fanatic
PS - The simplest solution is usually the best one.