वस्नक्रयविक्रयाट्ठन्

Operational rule vidhi

The affix ठन् comes in the sense of 'he lives thereby' after the words 1. वस्न 2. क्रय and 3. विक्रय being in the Instrumental -3rd case in construction.

Word by word (padaccheda)

Left context वस्न- क्रय- विक्रयात् pañcamī · ekavacanaSubstitute / term prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Pink letters are it-markers (anubandhas), tags dropped by 1.3.9. Hover or tap one for the rule that marks it, or try the it-letter finder.

Full reading

वस्न-क्रय-विक्रयात् ठन्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix ठन् comes in the sense of 'he lives thereby' after the words 1. वस्न 2. क्रय and 3. विक्रय being in the Instrumental -3rd case in construction.
This debars ठक् ॥ Thus वस्नेन जीवति = वस्निकः 'who lives by hire--hireling'. In the case of क्रय and विक्रय the affix is applied to the words separately, as well as to their compound. Thus क्रयविक्रयिकः, क्रयिकः, विक्रयिकः 'a trader'.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.