TL;DR version: See title, plus: My real budget is $150, and I'm trying to keep Total Cost of Ownership down, while maximizing mileage/TCO ratio and balancing both with acceleration, then speed.
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I realize practically all the threads here are for specific bike worth estimates, and apparently for bikes worth more than the $150 that's my preferred budget, but this felt like the most appropriate heading. I can tell you here though that I've put in a few hours of reading, comparing, and mostly learning about what to look for. Unfortunately most of the time looking at specific bikes < $250 was spent at Amazon and similar big stores, with a couple hours trying to figure out used on CL/eBay.

For my ~$150 preferred budget, I'm currently looking at the GMC Denali on Amazon, and, yes, I scanned a couple of epic threads arguing about its merits. I found 1 on CL, older year that may have some mechanical issues not resolved until the current version, for $100 asking, I figure I'd offer $60 assuming it's in as lightly-used condition as described, and see where bargaining goes from there. Although at $100, I feel like a new Denali for $160 shipped would be preferable to also get a 2-year warranty w/ help of my credit card extended warranty.

However, since I did see a lot of strong claims that used bikes can be had with better durability and performance for under $100, I wanted to get some help finding them if they exist in Los Angeles (or can be shipped under budget).

I understand that Craigslist, eBay, and bicycledepot are likely starting points, but after reading up on some $60-90 deals some people have mentioned, I thought I'd try and find something up to $150 within the next couple weeks. Thing is I'm having trouble filtering CL even <$150 for Los Angeles, especially with such brief posts and vague model titles (besides lack of model years). And on eBay, even if I could tell what types of bikes to look for, I don't know about throwing out bids on random bikes.

As for yard sales, swap meets, police auctions, etc., without the internet or other guide I'm pretty useless telling bikes apart for value aside from spinning the wheels, checking joints, and looking for rust; I have no confidence in picking out the bikes that will run 2 more years, 5 more years, 10 more years, etc. Pointers on makes would be nice, models and years that might fit my budget considering the Los Angeles market will be even better. Specific links to ads/listings to follow up on would go the extra mile and be most appreciated.
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I have < 3 hours experience mechanical experience working on a bike but I'm comfortable with tools and learning up on what I need, and I think I can get my hands on any needed tool, except maybe a truing stand, without renting or hiring out so I can DIY. I'm planning on using homebrew lube and marine grease to keep consumable costs down, but I'm also prepared to do daily ride wipe-downs, monthly cleaning via chain soak w/ relube (for 5mi max daily commutes), and hub maintenance every 6-12 months, and anything more involved every 12-24 months (although for cost of replacement parts, preferably every few years on anything but the chain, tubes, and tires).

That's possibly more maintenance than needed given my requirements of a primarily commuter/errand bike in a city predominantly flat with dry weather. Anyway, 5mi max daily commute on paved but sometimes poorly maintained flat roads, although it'd be nice to be able to range 40mi on the weekends in mild inclines and maybe light dirt trails in the parks. Acceleration would be nice since I don't expect to get a lot of freedom to build up speed, but I'd also rather go a bit slower than risk a car accident. I will likely put in a rack and maybe fenders, and I'm not sure about drop bars, partly b/c of city road visibility (from other cars as well as my own), but also because I've never ridden drop bars; I'm wondering if a trekking or moustache handlebar might be useful as I increase range. I'm 5'9" with a 29"-30" inseam to floor, and I will have 180-280 lbs load on the bike. The bike will be stored overnight in garage with makeshift fitted tarp against dust, but daytimes will be security-chained, probably in the sun/partial shade.

Thanks for reading this far.