can I still offer the 'buy now' price after bidding has started?

[*sparklz*]

No.

Once bids are placed the option to add buy now price is removed.

If you want to add a buy now then you need to end the listing and relist with it BUT once a bid is placed the buy now option is gone.

If you want a set price it is best to list as a buy now only.

[digital*ghost]

Once an item has bids, the option to Buy It Now disappears and the listing proceeds as an auction - there's no way to still use the BIN option on that listing.

A seller would have to cancel all current bids, end the listing and relist to offer an item at a BIN price, so most will let the auction run.