One of the things that I like to do is go to PCGS or NGC and scan through coins that are actually graded so I can make a comparison. One of the most striking things that I always notice is the sometimes vast difference in the appearance of coins and the grades that they have received. I believe the old adage is that if three "experts" look at your coin and offer a grade, you will likely find 3 different grades. Grading is very subjective and when it comes around to that fine difference between AU/AU+/AU++/BU it can be very confusing. The best way is always to be able to see the coin in hand, but if not, I would insist on unaltered pictures that are shot in a light box. There are a lot of really crummy sellers on here that like to alter colors of their coins and that can hide a lot of flaws. I would also NEVER buy from anyone that shoots their picures of a coin in coin flip and if they shoot it in a capsule then they should take the top off of the capsule so you can get a good look at both sides of that coin. If it is already certified and graded by an approved company and the certificate checks out - you should be good to go.
I believe someone else stated that you ultimately set the value, which is so very true. I have had many people decide that a coin was worth 20X more than what it books for in auction and that is great for me, and it is great for them if that was what they really wanted. One last thought - when deciding a grade and a price, I will often also look at what else is being offered on eBay and other sites and try to find some coins that look as close to what I am looking at or about to sell. You can find a lot of near twins, triplets etc out there and then gauge what the going prices are for the most similar looking coins from that year and mint.
Hope this helps you out.
