I agree with Diedrick I wouldn't worry too much about getting a buyer's list first per say... I did both sort of simultaneously, searched for both qualified leads and serious buyers at the same time.
I can only speak for myself and say that I am a novice so just take my word with a grain of salt. But I have a spreadsheet for my leads with relevant info that I try to have filled in before a potential seller calls. That way as soon as I answer and get their name, I can open my spreadsheet and see which house it is, bed/bath/sqft, neighborhood, built/renovation date, owner address (to know if O/O or absentee), if it's SFR/MFR/Condo, my estimated ARV, when I sent the letter to them/how many letters I've sent to them, and the general condition of the house from what I could see and find online. All this info by going to my county tax assessor website and sites like realtor.com and redfin for house info and value estimates, takes me about a minute per lead to get it all. I also have a separate sheet for each list, so I can tailor the conversation accordingly.
Having that info at my fingertips has saved me a lot of time when I actually do get a call, because I don't have to sit there and waste 20 minutes with "what's the address, how many bedrooms, how old is the house, do you live there" blah blah. I can just ask a couple of open ended questions ("tell me a little about the house" to which the response usually includes if/when improvements were made, why they like/dislike the house, and if they owe a lot on it... and "what are your reasons for wanting to sell" to which the response usually includes their motivation and how much they're looking to get). At that point we're only 5-10 minutes into the conversation, and the seller feels like we're having a friendly chat rather than an interview/interrogation.
I end the conversation by setting up a time to see the house. Since I already have most of the info, I don't need much time to gather intel. I can actually set a time within a couple hours to meet, which if you ask any salesperson the quicker you move the more likely you are to close. I take a few minutes to calculate my MAO from the ARV I already have, the usual assumptions for cleaning paint etc, and the info I got from the seller, and jot it down in my notebook, which I take with me to the meeting.
Then all that's left is to meet, go through the house and take note of anything that wasn't discussed on the phone (like cracked outlet covers, clogged gutters, faded siding, etc) and jot everything down in my notebook. By the time we sit down to talk I've already recalculated my MAO and we start negotiating on the spot.
I have not used a contractor at this point, however like I said I am still a beginner and I do plan to actually start getting inspections for each property I get under contract before passing them on to my end buyer. Having read J Scott's book on estimating rehab costs I feel confident and informed enough to go in without a contractor.