Considering that the grade of a book is absolutely paramount in determining its value, the fact that many sellers aren't prepared to provide one is a wee bit lazy, and in certain cases an attempt to game the system. Many sellers also simply say "I'm no expert - check the pictures" without leaving any notes whatsoever. The reasoning behind this (the occasional picky, difficult buyer) becomes the get-out clause and allows for such passive-aggressive selling - "I can't be bothered to supply the grade or any notes about hidden defects, work it out for yourself, dammit".
Some do come to the rescue by actually letting you know that there are defects not visible in the scans or photos, such as amateur restoration, or that the centerfold is missing - it does mean the potential buyer will move on, but at least a headache has been averted.
And then there's the old canard about grading being subjective. If that is entirely the case, then why have a grading system at all, that has been honed and refined over the last several decades? If you're going to sell expensive books, and enter a developed marketplace that has given you the opportunity to sell stuff that you may not have realized was valuable, surely a week or two of research, asking questions on forums, and making some kind of an effort should be the minimum?
Grading is not subjective. There are enough people on this forum and others like it, CGC and its competitors, dealers at cons, collectors etc. who understand the grading system which has developed consensus as to what grades mean. There are plenty of sellers in our marketplace who I deal with who generally get it right (there might be a half-grade disagreement, but rarely anything more than that), which is why I've thrown so much money away here.
Learning to grade and what grades mean is not that hard. At the very least a passing conversance with it, enough to state "I think it is this grade" would be nice.