But they said they still have fleas.At this point, I feel like it is fairly safe to say that they brought the fleas in with them or at least their cleaning habits (or lack thereof) have contributed to the problem.Anyway, my question is who is ultimately responsible in this case? 

Maybe the approach is to have it inspected so you can clear it up and also have them inspect.

Always approach the situation as a problem solver and a solution provider, everyone wins. 

My issue is that the seller likes the idea of deferring his capital gains over a few years and I don't know the best way to approach the scenario.

A year later I brought it up again, even offered to 1031 one of my higher value SFR in the same area.

As someone who may find themselves in a similar position (but with a spouse who can help augment the monthly), I would assume your two best options would be to either flip or take the buy and hold BRRR approach. 

I brought this to the attention of my lender who said he would look into it and work out the numbers.

What was your approach?

My question is, how do you approach a potential private money lender when you don't have much experience to show that you can provide a return on their money?

@Aditya Namjoshi - see notes below.1) The BRRR approach is typically used for your primary residence so living in SoCal and investing out of state isn't possible (I don't want to sound negative).