I wouldn't worry too much about insulting them. Odds are if they are insulted by any offer, even one they won't consider, they are too emotionally involved. Business is business and buyers make the market.
That said, there are lots of ways to figure out what might insult whom.
Do some research. Run the owner's name through the county tax/property records database. Look at how may properties they are into, if a builder how many completed properties they have sitting. Also look to see how much in hock they are to the banks of the subject and other properties. Look for tax or other liens. Prior or recent foreclosures. If it's just a single personal residence, check time on the market and other information. Even driving by to see if they have a newer luxury car will tell you if they're shelling out alot of cash and need out. Finally go by the courthouse and run their name through both civil and criminal indexes. Civil judgments mean money owed and criminal ones mean lawyer fees owed. There are lots of other ways to gauge their vulnerability and the bigger the beggar the less choosy they can be. All depends on how much work you want to do, but I regularly do this type of thing when working with a buyer to determine a good offering price. In some cases the biggest insult is making no offer. Personally I have no problem presenting low ball offers to my sellers either. We can always counter with a few thou under asking to let them know we're not biting but we'll deal. Nothing venture nothing gained and you can't get a deal until you get someone with offer to talk to.
Dec 13, 2008