Some property investors would like to buy houses at good location but in poor condition, to renovate and largely mark up and reap a huge profit from it. How could an agent persuades a buyer to buy a property which is owned by the vendor for a very short period of time and is being re-sold at a much higher price?

What is the value of the property after repairs?

That is the market value and it has nothing to do with who or when or what price a property was previously purchased.

Value is value. If the value is supported by the market, comps are available then that is the value. If the buyer wanted to assume the risk of purchasing and remodeling a property to increase its value then they should do that and not buy one already improved. It is really a no brainer.