for the best results, you should both respond to members' ads, and also post your need for points.
regarding etiquette, IMO initially, you can try contacting a number of members to see if their points are still available, what their asking price is, what their terms/conditions are, etc.
once you do find someone who you probably want to work with, it is only polite to work exclusively with that person.
it would be kind of rude if a member spends a lot of time and effort trying to book your reservation for you, and then you suddenly decide to rent from someone else because you happened to be working with more than one person at once.
keeping the lines of communication open is crucial. try to keep the member updated... for example, if you change your mind and don't want to rent from them anymore, do let them know so they can move on to the next renter.
also, respond to everyone who contacts you, even if it is a short note to say, "no thanks, i am working with someone else".
regarding how long it takes to setup a reservation, it depends on what your travel dates are, and how sold out the resorts are then. keep in mind that dates within 2 mos of check-in are usually sold out, and also GVs and Standard Views at BWV are also often sold out months in advance.
HTH! good luck, and i hope you have a great
DVC vacation!