Flip-Why Wont This Work?

[Andy More]

I know most here will rip this deal to shreds and says its too thin, but why? What am I missing. The property is a duplex in immaculate condition and rents for $1,500 per month. I know this neighborhood like the back of my hand, I used to own the duplex next door. The guy who owns the subject property contacted me and said he wants to move out of state and wants to sell his duplex, "make me an offer". I want to ideally hold this property for no more than 6 months and make a 20k gross profit. I could however hold indefinately if necessary.

165k(conservative current market value)
-800(purchae closing costs, paying cash, no mortgage)
-5000(realtor commisions, 165k is a low enough price i think I can get buy with 5000 in commisions, ideally sell without realtor though)
-800(closing costs at time of sale)
-0(repair costs, property is immaculate)
+4500(net rental income for 6 months after deducting 50% for expenses)
-20,000(desired profit from the deal)
=$142,900(max purchase price)

Awesome tenants, property is in good neighborhood. If I count the $4,500 in rental income I am buying property for 86.6% of current market value. I'm sure everyone and there brother will say NO DEAL at 86%, but why? Accordng to my numbers I'm getting my 20k desired profit so why shouldnt I buy it for $142,900 and flip it if my numbers are right?