but the operator should also be careful of not making the customer confused.

they can simply tell you that the item  is reserved by someone else, you may come back later or we will let you know shortly if it becomes available, but they can't let you have it and then take it away from your hand, :)

it is the same as buying a house, you might give  an offer on a house and claim to buy it, so others can only wait and see if you are definitely going to buy it, if not, they will start to bid, but what is guaranteed is that, the agent that wants to sell the house , as you said, surely doesn't want to loose either of the customers.

keeping it simple, will make it easier for both operator and customer to deal with it easier, that's how I look at it anyway, but I guess it is up to the implementation to make the nice and neat for the customer to encourage them to come back, by having services such as notification of availability  of the items,