@Devin Martin fundamentally if you want to flip, it means one of three things: 1) buying the property at a discount sufficient that you can flip the property and make a profit, or 2) buying the property at market at making sufficient improvements that you can flip the property and make a profit, or 3) both.
If pursuing the 1st then your goal with Listsource, or any other list provider is to create a list of "motivated" sellers who might want be willing to sell at a sufficient discount. These motivations range a great deal from financial issues (foreclosure, tax default), personal issues (death, bad relationship, hoarding, etc), to property issues (code enforcement, bad tenants, no tenants, etc). Listsource is useful for some of those, for others you'll either need to find other services, or gather the data yourself (pretty easy to get a tax default list in most counties for example, or to read obits in the newspaper).
If pursuing the 2nd with Listsource, or another, then you'd focus on searching for large/double lots (potential expansion or lot split), large homes with too few beds/baths, homes that are too small compared to neighborhood, etc. That last one works really well when there is a large discrepancy between cost to build per sf, and resale per sf.
Combining both strategies can be great too.