I was going to say the same buy low sell high that nlerner posted. One other wisdom I can think of is part it out! Bikes are almost always worth more in pieces than whole. I have made almost twice as much on the parts than selling complete bikes.
I have kept accurate records of bike purchases/flippers/part outs in a few Excel spreadsheets and old fashion paper. I have three excel spreadsheets for the bike "business". 1)ORIGINAL PURCHASE PRICE=what I paid for every single bike I have purchased (140 bikes as of now) 2)SOLD-two separate sheets on the same excel problem. First sheet is frames/forks or complete bikes sold with price it sold for, what it cost me to build, and profit plus date sold. the second sheet is parts with the same sold for, cost, profit, and date sold. The third sheet is all of those numbers added up by month for bikes/frames/forks and parts in separate entries. 3)CUSTOMER LIST with name, what they bought, and email address. It is handy for refreshing my memory if a previous customer is looking for another bike for themselves or friends or just parts I can recall what they bought previously. For the actual calculations of the cost to build up a bike after an overhaul I use old fashion paper in a three ring binder. I make a list of all of the parts and costs associated with making it rideable again. I only list the new stuff I add (bar tape, tires, tubes, cables, housing, etc.) not used parts I use from the parts bin. I also make parts lists when I part out a bike of the parts that came from that bike so when they are sold I can enter the numbers and figure how much profit I made parting it out.
If you really really want to be a flipper you need lots of cables, housing, bar tape, tires and tubes and to get them cheap. I work at a shop so I restocked my cables and housing this summer with the 50m rolls of housing and 100 count boxes of cables. For tires, tubes, and bar tape you can often find deals on Nashbar or Performance when they put stuff on sale with an additional 10-20% off coupon. When they do stock up! The last time I stocked up with a few hundred dollar order I bought 20+ tires, 10 things of bar tape, and 20+ tubes. You also need the know how and all the required tools to do it which can be expensive on their own.