Property valuation way less than list price.What should I offer?

[Sello Mkantjwa]

There is an apartment  that I am very interested in purchasing in order to rent out ( my first purchase).  Initially i thought that the list price was reasonable and I could negotiate it down by about 5%. However I purchased a report that gave the valuation at about 15% less than the list price. Identical properties in the building were also selling for around same price as the valuation, last one having been sold in August of 2015. No property in that building was sold at the asking price. All purchases have been far below. 

My question is, where should I start my offer? Is it normal for sellers to mark up their prices so high? 

Thanks, 

Sello