agent wont ask valuer to value the house.
agent will sell based on what you value your house.
So give an example, you bought a house 10 years ago at 200k

now there are people selling 500-600k
you think your fengsui good, well maintain, no negative element, you want to sell for 800k.

agent cant stop you. but if it is over price, hard to find buyer. the agent will promote those of lower value one. ( higher chance of success with less effort)

Say- you are lucky and one buyer really like your house.
he need to get loan, - that is where the valuer come in.

valuer is arrange by bank ( after he sign letter of offer) and is to be paid by buyer. if buyer getting 90% loan, then the valuer report need to state the house value is 800k or so. Else the bank wont approve the loan, or approved at lower margin.
in short- they want to minimize the risk
of course, another factor is RPGT.
if untung, and less than 5 years then RPGT apply.
if hold more than 5 years then don't need to pay.

but in your next e-filling, need to enter details on the section on "lupus hartanah" or something. i forget the detail liao