I would use
www.discogs.com to check out how much in demand (people add items to a wants list) the vinyl is, or see the current market value that some people are charging using the Marketplace.
Can't remember off the top of my head but I think some people have bought/sold whole collections there as well.
It's pretty easy to sell vinyl on Discogs, you basically add the vinyl to your own collection then say you want to sell it - saves so much listing information - although be sure to get the record catalog number, label, issue etc. information correct.
As for mass postage, one of my mates deals in dance Vinyl collections and swears by
http://www.mhdirectcouriers.co.uk/services/ - basically DHL but very cost effective. I've used them for big parcels/heavy items and they're miles better and more reliable than using any other service, bearing in mind this saves you lugging the Vinyl to a post office or other such location.