I'm a copy-editor - not developmental - and I'm always as clear as possible with authors upfront about exactly what I'll do. I do them a sample edit, as well. I use this to price up the full job based on an hourly rate - but I insist on having sight of the full manuscript before doing the sample edit. This is so that I can have a look through and check that the quality/standard is roughly the same throughout. If it is, I'll pick half a dozen pages at random for the sample. This avoids the problem of the author sending me their 'best' pages, or just the first chapter where they were concentrating properly ;-), or a bit that they've already had edited. I've had some books that weren't the greatest, admittedly, but never any real doozies that way. I like to think I'd do the ethical thing and suggest beta readers/a developmental edit/whatever they really need.