She told us that she was giving us 30 days notice and that she sent a check for $300 and to apply her $800 deposit to the last months rent. 

Who paid the deposit/equity in the first place?

This payment shall apply as an additional deposit.  

The strategies in the book are not designed to work with the seller who insists that youre preapproved and have a large deposit and good credit.

Their schedules are so unpredictable that I now require a deposit.

I have bought 2 houses on credit cards where I can get up 30+k direct deposited to my checking at about 5.9%, but I'm looking for other possiblities.

The deposit would go right in your pocket.

AS to why you need that long, inspections, partners approvals, more inspections, this IS an investment, you need proper time to inspect.As far as the assignment fee, you get that after closing, though part of your assignment fee can be charged before to the seller as their non refundable earnest money deposit.

And if I am doing a deal that requires money down, wouldn't I just have to use an earnest money deposit?

The iFlip system along with his "Fast Cash with Foreclosures" system run about $1,000, but it's like a deposit.