sell (the) most coffee

No, it's not really optional. But it's also important to know just what you are trying to say here -- that your shop sells more coffee than anything else or that it sells more coffee than other shops sell? If it's the former, then it should read "Our shop sells mostly coffee" (no 'the' in this case).

The usual rule is that superlatives (most, biggest, tallest, longest etc) are preceded by the definite article ('the'). It is customary to add a qualifier.

Mexico City is the most populous city in the world.
Of the countries of the world, Monaco contains the most millionaires.

Of all the shops in our town/district/country ours sells the most coffee.

However, 'most' can mean "the majority of", and in that case 'the' is not used.

Most British people drink tea.

Most people wear clothes.

Thanks, Andrew and Contrex.

Something for which most of us are profoundly grateful.