Where to sell a rare coin with a mint error?

RFW

Krakkkers said:

Because i have sold over £5000 of stuff on ebay so far this financial year.
The coin was given to me in my change at asda and appears to be worth up to £200

Sort your tax out by all means but if you have a 50p coin that's worth £200 you'd still be quids in if you did pay tax. This is a personal sale though so you shouldn't be liable for any tax anyway.
If the coin is worth £200 and you are a business seller I'd recommend using the windfall to pay for an accountant. You may find a decent accountant would advise you on your tax liabilities and that would be less of a worry for you going forward.