For quick-flips, consider domain industry market cap over anything else. Yachts sell for much more that paperclips, for example. I would avoid food-industry and religion-oriented domains entirely, unless you're lucky enough to own a domain like Jesus.net.
Next, consider keywords search volume and PPC.
Other factors like # of TLDs taken, age, etc. are tertiary factors you could employ as bargaining chips.
IMO, only purchase domains for which 4+ highly targetable, well-established end-user candidates already exists. If XYZ.com has 3 potential end-user candidates and the best-off one generates only $50,000 in sales per year, XYZ.com is not going to sell for more than $150 or so. Your quick-flip earnings are bounded by how much your potential end-user candidates are (1) able and (2) willing to pay (whichever of the two amounts is smaller).