This is an old link I found but serves well for our example) Let's assume that at any point, 10 percent of this inventory is for sale (no data found on this but any number should work).

You can buy that data from list brokers right here on BP 

That is always a must, and  then approach every owner the same with the same info and data. 

For a 10k gain in value...

Being a landlord to your friends is much harder than i thought, they assume that friendship will come before business, which does, unless you're refusing to pay the rent. learning to seperate business and relationships is invaluable

Being a landlord to your friends is much harder than i thought, they assume that friendship will come before business, which does, unless you're refusing to pay the rent. learning to seperate business and relationships is invaluable

The Agents have all the data to get you oriented on the current situation. 

What caused the drop in value?

If your realtor doesn't have a sophisticated system for querying the MLS a lower-tech method is to have the agent do a full CSV dump of the MLS for SFR and send it to you after stripping out the non-public data columns. 

@Przemek Kos, I would not judge offerings on one data point.