The affix ṭhak comes in the sense of 'this is whose saleable commodity' after a word in the Nominative 1st-Case in construction denoting the thing to be sold.,
Thus अपूपाः पण्यं अस्य = आपूपिकः 'a cake-vendor.' So also शाष्कुलिकः, मौदकिकः &c.,
तद् अस्य पण्यम्
Adhyāya 4 · Pāda 4 · Rule 51