The buy average remains unaffected when shares from holdings are sold and bought back on the same day (excluding T2T stocks). Intraday trades involving shares from your holdings are treated as separate transactions, and the buy average calculation doesn't consider them. This is because the shares don't physically move in or out of the demat account during intraday trading. However in the case of T2T stocks each selling and buying will be separately settled with delivery resulting in change of buy average price.