In the krayya there is substitution of ay for e when the sense is that of 'exposed or put out for sale, saleable'.,
The word क्रय्य is derive from क्री 'to buy', with the affix यत्; the guna ए being changed to य ॥ The word तदर्थ means 'for the purpose of that' i.e., for the purpose of being bought. As क्रय्या गौः, क्रय्यः कम्बलः ॥ Why do we say 'when the sense is that of saleable'? Observe क्रेयं नो धान्यं न चास्ति क्रय्यम् \we want to purchase corn, but it is not put out for sale\.,