The post read a little awkward at times, but overall they give a good 'gist' of how to select a good copywriter.
I fully agree with several points; real copywriters don't just write, they're marketing strategists. Excellent copywriters can also give you strong layout ideas (I'm not saying they should do design work for you, but they should be able to tell you how to format/lay out your piece to effectively present the copy)
I also agree that you should never hire a writer that 'guarantees sales.' No one can guarantee sales — there are just too many factors involved.
Another tip: check how busy a copywriter is. Copywriter posers often can do work for you at the drop of a hat. That's because they have no projects going on... not a great sign.
You might also want to talk to a copywriter a bit about their process as well. If they mostly talk about writing, there's another red-flag. A huge part of copywriting is research and brainstorming.
Bah, I could go on and on about copywriting
I'll stop here... (for now).