Generic or Official-looking Letter of Intent

[GREG B]

I'm trying for the very 1st time to wholesale a REO house. I submitted an offer with a real estate agent who showed me the property. Within an hour of my hand-delivered Letter of Intent that I gave her, she called me back and said the offer was denied due to my offer being too low, and that I also have to use an official TREC form (Texas Real Estate Commission)...and in addition to that--I must also submit "proof of funds".
On my original generic Letter of Intent, I made sure to indicate my terms/payment would be ($ 79,450 in CASH) and that closing shall occur on or before 21 business days.

--Should I have stated closing to occur on or before 7 business days (since my buyers are capable of closing fast)??
I also left the offer "open" for 4 days--Should I have only given a 48 hour time limit in which to be accepted?? And furthermore, how would I obtain proof of funds if I'm using private money lenders/wholesale buyers/investors?? OH, yeah--do I "have" to use a real estate agent to submit offers?--or can I just go strait thru the bank's reps./managers??

NOTE: House was foreclosed on for $74K-- listed for $119K, then dropped to $99K--on the market for 90+ days. So, yeah I put in an outlandish offer of only $5K more than what the bank paid for it.