If you've got the front and back plates then they are worth quite a bit of money, you can buy level 4 ballistic plates online for around $70 - 140 per plate in the states depending on the size, to give you a rough idea. I don't know what to sell them for from the Osprey though, as I've never seen any for sale.
It's finding someone to buy them that'll be the hard part, because they are illegal to own and a lot of people know it. Couple that with the fact that they are utterly useless in airsoft, and very few people want to handicap themselves to that extent. It's a lot of added weight for nothing.
The soft armour is also illegal to own... I have a set of side plates as well as the soft armour, the Osprey I bought came with them in it, didn't even know I was getting it. I also have a Mk7, because I bought it without knowing they were illegal, and obviously the bloke selling it wasn't going to tell me...
Mk7s sell for between £60 and £120 though, depending on how little the buyer knows about them, or how desperate they are to get one. I paid £80 for mine quite a long time ago, but I was offered a few for around the £120 mark prior to buying the one I ended up with.
It ought to be easy enough for you to price up the other stuff by searching for it on Ebay, Mk4s go for anything from £40 to £200+ depending on how much of the extra bits and bobs they come with. All the extra little shoulder blade and upper arm covers and everything, as well as all 13 issue pouches will net you £150 or so I reckon.