Are you positive this is the 1768 edition (the first year published)? According to multiple web sources about Britannica, the first edition had only 3 volumes (published in Edinburgh, Scotland); you say your edition as 30 volumes.
Take it to an appraiser. Not an appraiser who wants to buy it. You will get a low price. Take it to one who will appraise it for insurance purposes and you will get a more accurate value. A jewelry store could probably put you in touch with one.
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