You are not really renting points - that's just how it has come to be named. The owner is really renting a reservation that he/she made for a renter. So the "minimum" would be enough points to make at least a 1 night reservation.
Some owners are also willing to transfer points to another member. According to the POS, owners are not allowed to charge for this. In practice, as long as you do not mention money to MS, they look the other way. However, DVC/MS/Disney will not get involved if there is any dispute over a transfer. Transfers are final - they cannot be "undone". Arrangements are between owners and entirely negotiable. Members can transfer or accept as little as one point. This option is only available to owners.
How long does it take? Really depends on finding a renter and then the agreed upon terms. It can vary from a few hours to never. Price also makes a difference.
From what I have seen, there can be quite a learning curve for new owners who begin to rent. Many seem to "learn the hard way". Nearly all of the reported problems are the result of one or both parties not understanding how DVC really works. There have been a few "scams" - a few times the owner lost out and a few times it was the renter. Scams are very few and far between for those who use this board to arrange the transaction and do the proper due diligence before sending money.
I also do not recall any $$/point losses experienced by our posters who have chosen to use either of the DVC Boards sponsors to rent or sell their points - David's or TSS.