Make sure that are not "added." I have not seen one of these in over a month but if memory serves, I think the reeds on these coins should be cut at an angle. Someone here will know for sure.
Just weighed it and it is 8 grams. My scale doesn't read in between grams but I know the that the real Guinea should be 8.4g. I'm pretty sure it is real just don't have a clue about grading coins
Then he considers it genuine and he has the coin. He will probably sell it as a VF/XF (American grade) price.
Take the money from the dealer. You could ask £270 to make an extra fiver. If you auctioned it, it would nett you less after costs. It appears to me to be VF, not unusual for the issue. Although the gold is soft, they got much less handling.
Easiest is to sell it there. I have know idea what it is worth. In England it probably grades Fine. Get another offer to see if the quote you got is reasonable. The coin my be cleaned, or have tiny scratches that affect the "true" grade. Glad to hear it is probably genuine. Good Luck.
