Normally a 10% deposit payable to PR is to be expected. 

If they were really hellbent on getting a deposit, and if I was looking at a nice spread that I didn't want to pass up on, I'd be willing to give them a small deposit.

There is nothing that erodes trust more than proceeds that don't match the offer.  

(footnote: they don't believe in email. so it wasn't an option.)THE SOLUTION:After spending the evening thinking it over, I figured I could offer a less "daunting" offer by presenting an "Option to Purchase" contract instead of a blatant "Purchase and Sale agreement with assignment clause", which would leave them locked in with me for 30 days.The difference in these two contracts (from my understanding), is that an "Option to purchase" contract would allow the seller to still sell the property to whomever they wanted, while I would also simultaneously be able to market the home at 0 cost to them for 30 days.If a buyer approached them with an offer that was higher than the one in my option contract, they would give me the opportunity to "match the offer", or I could simply walk away from the deal and they could go with the higher price.THE FAIL: As I was fighting traffic demons to get to their property by 3PM, I gave them a call to let the seller know I'll be there soon.

A good site to use if you want to do your own is redfin.com When you run a search, filter the criteria and on the right side of the page it will list all the properties that match that, at the bottom of that list is an option to "download all" and you can export the file to a excel document and run a comp analysis

What sort of qualities  should I look for in them to know they would be a good match for my investing career?

All tenants' deposits have been taken as well.  

I'm not sure if I should ask for a non refundable deposit or just wait until I assign the contract to them.

Sounds like a wholesaler if they want a deposit upfront.Question #1- Are you a wholesaler?

Her behavior made us decide to offer her the entire security deposit back if she and her family would vacate the premises by the weekend and find a house that suited them better.