Jason's advice is sound. I figure there are a few ways to tackle your situation:
1) Learn about coins, enjoy the hobby, and concentrate on what you'd like to collect and sell the rest.
2) conduct some research (internet auctions, books, coin shows), find out what is rare or semi-rare, put them aside and attempt to give them a rough grade. Also put aside those coins that stand out - those with eye appeal. Sell the rest unless you want them in your collection. Conduct additional research on these - ask the best place to learn about these or where to sell them.
3) find someone that you can trust - easier said than done.
4) Then there is always Ebay.
All of the above require some work by you. Ask questions - enjoy and learn.
as far as your 1817 half-crown, you have a type 1. See the approximate retail price below (in pounds):
“GEORGE III.925 fine silver, 32mm. Weight 14.1g
Date ESC Mintage Fine VF EF UNC/BU
1st ‘Bull’ style head
1817 616 8,092,656 £20 £65 £250 £450/
1817 616A ‘D’ of ‘DEI’ over ‘T’ £50 £200
1817 ND E of DEF over R&E £400
1817 ND S of PENSE over I £200
2nd Type, with re-designed smaller head and new reverse.
1817 618 Included above £20 £65 £300 £500/
1817 618A S’s in Motto mirrored Very Rare
I would grade yours (obverse) Fine, but would have to see the reverse to give you an accurate grading and to see if you have a rare one or not. I am conservative in my grading.