There are two (at least)  affordability and availability (inventory) .

They won't sell because they are afraid they won't find the next home to buy.

TIME! Internet marketing is essential BUT getting out and "pressing the flesh" is the best marketing if only one can get out from behind the computer SmartZip Analytics .

The challenge of finding a replacement property if they are not moving out of the area. 

Things for them to buy next.

The biggest challenge for me in our area is the fact that there is such a LOW INVENTORY that the Sellers are hesitant to List their Homes because they are So Confident their house will sell in 1-2 Days (and they are probably right) ... that they will not be able to find suitable housing.  And they are not willing to take "second best" ... so they are deciding to just "Stay Put".  IMHO

there are too few of them selling.... 

Many of the listing agents we work with find low inventory to be the biggest challenging / frustrating obstacle, SmartZip Analytics. So, definitely echoing what Diana Dahlberg mentioned. However, our listing-generation technology (Seller's Corner, more specifically) that we equip folks with successfully combats those hurdles, head on. - Tim

I dont see any obstacles, just keep going forward as fast as I can.

Smart Zip,

I agree with Michael Jacobs 

Been some time since I actively prospected. Some potential sellers are hesitant because they fear they will get caught short replacing what they sell.

The limited number of available properties can lead to buyer frustrations.

Finding a place for them to move to.  Most seller's need a landing place, but many also need their equity. We talk chicken & egg all the time, which comes first, buying or selling.  

Too many people don't want to sell because they don't know where they want to go and afraid they will sell and be homeless!

Friend and relative interferrence

Nearly every homeowner is a potential seller. The approach to sell is the key

Not prospected for sellers.

The lack of between balance of common sense and terms.

Not finding a replacement property.  With our strong Seller's market, it is a frenzy out there.

Unpublished Phone #'s

I don't get frustrated very much.

I can sell their house in no time but... where are they going?!  Inventory is low and many sellers need to seriously consider "transitional housing" while they put themselves in the best negotiating position &  wait for the right property to hit.

Getting listings is no problem. You only want the ones that are priced realistically. Not the guys or gals that start to shout "not going to give it away" when they don't really want to sell but would for way way above market value. The seller's remorse is hard on ROI but it happens. Real estate is a numbers game. Listings are king.

Showing value to sellers who only want to look at agents as a commisison they want to scrape to the bone.

Let me guess, targeting sellers before the decide to list?

Inventory is the biggest obstacle right now. Buyers can still afford homes if they can find them.

Of all the hot-shot "listing experts" running about the real estate world nowadays I can't single out any who have made a difference in the true inventory of homes for sale. They might get a better market share but the actual INCREASE is not forthcoming. You've all said it loud and clear! Don't want to sell because where am I going to go. 

FEAR and Expectations Disconnected from Reality

We are experiencing a low volume of inventory and there would not be much available for them to look at in the way of purchasing another home.

     News stories on TV, WSJ, NYT etc. that talk about the shortage of houses, skyrocketing prices, etc.  Also HGTV "Reality Shows", and Zillow Zestimates.             Here in Tallahassee we have more than enough inventory, especially over $300K.  When you talk with potential Sellers about the reality of our market, they argue, "but I read in USA Today that..." or "but I saw on that TV show..."

Bob Crane nailed this one!

Nothing, SmartZip Analytics.

Michael said it perfectly.

Good question and great answers. I hope it helps you.