MARTIN, or MARTYN (JOHN), bookseller in London; Bell in St. Paul's Churchyard, 1649-80. Partner with James Allestry, and succeeded him as publisher to the Royal Society. The last entry to him in the Term Catalogues is in May, 1680.
---A Dictionary of the Booksellers and Printers ... H.R. Plomer, 1907.
"A bookseller in St Paul’s Churchyard (according to L&M’s 1668-9 volume) or at Temple Bar (according to L&M’s Index)."
It's quite possible for both entries to be correct: the area around St. Paul's burned down in 1666, and so John Martin subsequently moved his business to Temple Bar.
Since Pepys first mentions Martin's Bookstore in 1668, we should assume the business was then at Temple Bar.
Chart showing the number of references in each month of the diary’s entries.