Treat this like a business, and be 100% transparent. If you do this for a quick buck, you will fail.
I always hold the contract for 30 days. I don't think you should ever need more than 30 days to move a house if you have a good deal. My contract is 6 pages long. I like to be as open as I can and make sure that the seller is on the same page as me.
For my buyers, I don't ask for proof of funds. I know who's reliable and who isn't. For example, I had a buyer call me Friday night asking to see the property I have. He told me he would call me at 11am on Saturday. I heard nothing from him, so I called him, and he told me he would call me around 11am today (Sunday) and I still haven't heard anything from him. Do you think I should even bother with him?
For me, honesty is key in this business. Tell the seller what your intentions are from the beginning, you would be surprised how many folks will let you find a buyer if you are upfront and honest with them. Of course there are some that have major doubts, but you can just follow up with them every couple of weeks to see how they're doing.
TREAT WHOLESALING LIKE A BUSINESS. There is a reason why 90% of wholesalers never close their first deal.