But, if you want to be nice, give them a warning after the first one then start the process the next time it occurs.  6 times in a row means it's probably time.However, if they pay every month and you are collecting your late fee as well... 

Some will collect first, last, and a deposit equal to a month of rent.

After this tenant leaves, you must clean/repair, market, screen applicants, select an applicant, collect the deposit and rent, get them moved in, etc.

For no/low-cost seller leads, I'd also recommend FSBO via zillow or realtor.com.As far as commission structure, the post-NAR settlement rule is that you are only allowed to collect the amount negotiated on your buyer agreement.

Also, focus on 2 years of job/income stability.Class D Properties:Cashflow vs Appreciation: Typically, all cashflow with little, maybe even negative, relative rent & value appreciationVacancy Est: 20%+ should be used to cover nonpayment, evictions & damages.Tenant Pool: majority will have FICO scores under 560 (almost 30% probability of default), little to no good tradelines, lots of collections & chargeoffs, recent evictions.

Also, focus on 2 years of job/income stability.Class D Properties:Cashflow vs Appreciation: Typically, all cashflow with little, maybe even negative, relative rent & value appreciationVacancy Est: 20%+ should be used to cover nonpayment, evictions & damages.Tenant Pool: majority will have FICO scores under 560, little to no good tradelines, lots of collections & chargeoffs, recent evictions.

It could be possible they tried to collect rent, and with the tenant knowing she's leaving, chose to not pay.

There are two idea I’m batting around in my head right now to get started on my REI journey – I can do it purely on the investing side and become a private lender or a source of hard money loans to builders/developers (maybe that looks like funding a construction loan collecting interest payments while buying time to better understand the dynamics and build relationships) or I can JV/partner with a builder and get equity (upside on a flip/sale) which would be more hands on.Curious what ya’ll input, what you think is the best route for me to dip my toes into REI given my aforementioned situation and current market.

Good chance they don't pay the owed amount, but you can probably also count it against their deposit and then send the balance to collections if they refuse to pay. 

Note I have no illusion that this path will create returns close to equivalent of what I used to be able to achieve via brrrr (I have infinite return on most of my existing portfolio) 2) sophisticated partition via coop/Tic and maybe CA SB9 on my OO.