As a fellow data scientist and former Craigslist/eBay item flipper and deal site (SlickDeals, FatWallet) hobbyist, my tidbit of experience and Sidebusiness 101: profit comes when sellers don't care to maximize due to circumstance.
Those circumstances include lack of product knowledge, urgency to move item, and value for their own time/convenience.
For the first problem, people have tackled it with eBay/Craigslist listing misspeller searches or just manually emailing for more info. I once bought a new sealed copy of a collector's game, Panzer Dragoon Saga (eBay price ~$500+ back then) for $20 from a seller who was liquidating an old stash of games and listed the item as "Panzer Dragoon Saturn" and couldn't get any hits.
The problem of illiquidity has some fun implications assuming you're willing to take some product risk. Back when I was in grad school in the Bay Area, I was flipping Fujiiryoki massage chairs (MSRP 5k, buying around 1-1.5k, flipping around 2.5-3k) and there are plenty more items out there. This requires either good product knowledge (tons of old Chinese people who I knew wanted these) or a data-driven way of identifying high-price-point items without much/any historical pricing data. This is a rather solvable problem that hasn't been tackled (that I know of) and can be a value point for your site I'd think.
As for buying liquid items low, subscribing to CL RSS feeds with the right search terms can do the trick, though I neither confirm nor deny that I may have once written a CL page refresher to buy Burning Man tickets at cost (instead of from scalpers who were charging hundreds more). Back when I didn't really value my time, offering to pick up at a time of their location/convenience and pay cash.
Misc selling tips as an enthusiast: and legit work/school email or Googlestalk-able email establishes you as a "real" person. A phone number is almost a must-provide these days too. Pay low(er) and always in cash (or cert. check at bank) but assure the buyer that you'll be there and make the transaction convenient and easy.
Seems like I missed the edit deadline. More free advice: Most flippers derive market price by using pre-existing marketplaces data like eBay/Terapeak and Amazon new/used to figure out where the market roughly sits. These venues have strong incentives to regulate listings into consistent item buckets to make sure people can't (CL)SEO listings like CL spammers ("2011 Macbook Pro" vs "2006 Macbook Pro with Office 2011", or the giant paragraph of hot items in small text and white font to catch your search attention). Once you get that magical price point/band, you can pretty much go to town with some basic clustering analysis. [Disclaimer: worked on a few private, weekend projects in this specific niche.]
Those circumstances include lack of product knowledge, urgency to move item, and value for their own time/convenience.
For the first problem, people have tackled it with eBay/Craigslist listing misspeller searches or just manually emailing for more info. I once bought a new sealed copy of a collector's game, Panzer Dragoon Saga (eBay price ~$500+ back then) for $20 from a seller who was liquidating an old stash of games and listed the item as "Panzer Dragoon Saturn" and couldn't get any hits.
The problem of illiquidity has some fun implications assuming you're willing to take some product risk. Back when I was in grad school in the Bay Area, I was flipping Fujiiryoki massage chairs (MSRP 5k, buying around 1-1.5k, flipping around 2.5-3k) and there are plenty more items out there. This requires either good product knowledge (tons of old Chinese people who I knew wanted these) or a data-driven way of identifying high-price-point items without much/any historical pricing data. This is a rather solvable problem that hasn't been tackled (that I know of) and can be a value point for your site I'd think.
As for buying liquid items low, subscribing to CL RSS feeds with the right search terms can do the trick, though I neither confirm nor deny that I may have once written a CL page refresher to buy Burning Man tickets at cost (instead of from scalpers who were charging hundreds more). Back when I didn't really value my time, offering to pick up at a time of their location/convenience and pay cash.
Misc selling tips as an enthusiast: and legit work/school email or Googlestalk-able email establishes you as a "real" person. A phone number is almost a must-provide these days too. Pay low(er) and always in cash (or cert. check at bank) but assure the buyer that you'll be there and make the transaction convenient and easy.
It's an addictive hobby, I know... :/