Vinyl generally survives mailing pretty well, provided you pack half way decently. I once received over 1000 78 rpm records via US post, they were poorly packed (standing on edge, no sleeves, some 75-80 per box, just "padded" with newspaper balls) yet only 4 or 5 were damaged! I've been a convert to using the US mails as my carrier of choice ever since, and some 3000 records and one complete audio system since, they've never yet let me down. Which makes all the noise about UPS and DHL and Fed Ex kinda funny to me, but that's several other threads, eh?
There are a few sites around that detail how to grade vinyl, I generally use the standards at http://www.post-age-collectibles.com/records/grading.html, but there are many others - just search for "record grading" on your favorite search engine and you'll see what I mean. In my opinion, the best thing is to look at a few guides, then grade conservatively from there.
You'll probably get better bucks by auctioning the records than by selling to a shop. Dunno if Audiogon is the best place to auction them, I love this site, but I think for the broadest exposure (and probably the best price) you might want to consider (ack!) e-bay. People on this site know music and seem pretty aware of what things are worth, while on e-bay I often see people happily pay ridiculous prices for things. The caveat is that you can expect to have more hassles with the unwashed masses of e-bay than from people who have some wits to hand. In any case, for maximum buck you'd sell each individually, but that's a pain in the place you don't want pain. Less hassle, but less money (usually) is to sell as a lot. Then, you're only dealing with one buyer, and making one trip to the shipper.
Hope this helps!