Once they get around 2.5 to 3 weeks late I get a bit uneasy... not that they are unwilling to pay the late fees, but at some point they won't be able to get caught up, since they clearly aren't good at managing money. 

It tends to make potential buyers uneasy when tenants are unwilling to cooperate, and I've found that simply being up front with them almost guarantees smooth communication and cooperation between all parties.

They had planned to rent until securing jobs but found landlords and property managers were unwilling to work with them because they had interviews lined up, but weren't employed yet.

It is harder to sell a property if the tenant is unwilling to show, but you have to first approach the tenant to determine if that's the case or not.

I've seen homes that were completely gutted inside, liens pop up at the last minute, and 'vacant' properties that still had an unwilling to move owner.

I personally would have only cancelled if the seller was unwilling to negotiate and I deemed the repairs too difficult/costly in relation to the acquisition price. 

Are experienced investors going to be in the business of "rental portfolio flipping" of these assets to new bright-eyed bushy-tailed real estate investors who are unwilling to go through the experience of putting their own portfolio together, and just want one ready-made, an extension of the idea of buying turnkey properties one-at-a-time every year or so, if you would?

If the buyer is uncomfortable or unwilling to provide proof of funds to you that may be a red flag; see if they would be willing to provide it to their attorney who can verify, and then make sure the contract protects you. 

If that's the case, then he might believe there is a high probability that you will be scared away from the property by a certain type of issue (and rightly so), even though he or she is unwilling to communicate those explicitly in the listing.Bottom line - I think you follow up regularly and try to get a good idea for why the seller is averse to you using traditional or FHA financing - perhaps with a starting guess that there are some skeletons that are likely to be uncovered in inspection and due diligence.I'd strongly suggest that you do not turn to another source of financing to close the deal on this first purchase.

My concern is that if I make an offer unseen, I may not offer the right price, or during home inspection the house is in terrible condition and unwilling to go down on price and I would have wasted time and money on home inspection and any deposits.