There are a few problems with this strategy.
1) You might be overpaying for the house if they have a lower figure in mind. If they have a higher figure in mind, you are not going to get the house.
2) No estate agent will believe you. They will be used to this sort of talk and will tell the seller that this is common place and that it's really just your first offer.
Negotiating the price is a game with a few ritual steps in it.
You put in an offer which is below what you are prepared to pay. You know that. The seller knows that. The estate agent knows that.
They will usually reject that offer or say that they will think about it or tell you that they now have a higher offer, whether they have or not.
You offer a bit more.
Knockout, take it or leave it offers, do not work in this market unless the property is worth about €500k and you are prepared to pay €700k because you want this house and don't care about the price.