If shopkeeper in his shop puts a vase in his window which states- Offer for sale for £500. Since goods in a shop window with prices attached are invitations to treat, this also is a invitation no matter if it states ‘offer for sale’ ( Fisher v Bell)
Adverts are VERY RARELY offers! The exception is in a ‘reward’ situation where a reward is advertised for (for example) the return of a dog that has gone missing