I buy and sell to perfection (5)
I believe the answer is:
ideal
'perfection' is the definition.
(I've seen this before)
'i buy and sell' is the wordplay.
'buy and sell' becomes 'deal' (I've seen this in another clue).
'i'+'deal'='IDEAL'
'to' is the link.
(Other definitions for ideal that I've seen before include "To which one aspires" , "Ailed (anag)" , "Model of excellance" , "Cutting/stabbing weapons" , "Standard to aim for" .)