Just posted my item for auction without buy it now price
within an hour i got email asking if I would consider a
buy it now price - I am considering it but how can i change
my listing, and is it good practice
You can add a BIN price if you wish, but it will disappear if anyone bids.
Or you could change your start price to the agreed price then when buyer bids you end early to sell to high bidder.
Or you could just let the auction run.
People often do this when they think they may get item cheaper than it would go for if left to run,