What and how could I negotiate with the seller to make it attractive for both parties?

I will have to do some negotiating somewhere because the seller is asking a price that is $15-20k above the comps in the neighborhood.  

If you have negotiated a lower price than what the tool tells you your purchase price is, you can adjust your profit, or add more into your contingency for rehab costs (this is where I tend to apply it).  

If you consider that your future business decisions will all be based on negotiations of some sort and you get emotionally involved to what better suits the other side rather than your own life and that of your families you are sure to fail.

Are you okay for this to potentially take 3 to 6 months (depending on the friendliness of your landlord laws) of your time at the purchase price you have negotiated, or do you need to ask for the Seller to make a concession to take care of this burden?

The alternative to a short sale is losing their home and getting hit with a foreclosure and the bank possibly coming after them for the full amount of the deficiency amount.In a short sale, that can all be negotiated.

While alot of Realtor hate the practice, I put in multiple bids to take the property off market, then have an inspection as well as a financial out if my due diligence does not meet my expectations or I negotiate a better deal on the other property. 

I found a 9 unit that the owner is asking 199k but said it is definitely negotiable. 

This forces lease re negotiation.

In fact, the wholesaler is the one doing the work finding places and negotiating and should be rewarded for that.Hope that helps and good luck!