That you have access to the line of credit in an amount sufficient to cover the price plus closing costs.
That the line of credit has been approved for use to purchase the specific address and under what terms (the bank will probably want a lien against the property).
The letter would have to have the bankers contact information on it so that I could contact him or her directly for confirmation.
I would want the buyer to put up a very sizable amount of earnest money and to have no contingency on financing approval.
Those points would be a good start, and unless I were presented with all of the above I would probably advise the seller to not respond to the offer until they were presented.
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