I mostly have experience with
ebay,
amazon,
crutchfield, and newegg. I wouldn't buy used speakers or amps unless I was comfortable and had the means to recone them/open them up. Open box in my experience has been good from each of those sources. Amps have a bad tendency on
ebay to be altered internally if they're not labeled new/open box. Even factory refurbished is dubious unless it's sold by an authorized dealer of that brand. Unfortunately there's a lot of bad sellers on
ebay out there ruining it for the honest folks trying to sell used gear.
Enclosure subs are a maybe, but they're often prefab boxes that came with the subs originally. It's a good value in the way that you can save a lot on shipping by getting them locally and a lot of people don't want to hold on to enclosures they're not using since they're big, so they'll give decent prices. Custom boxes are virtually always better than pre-fabs. I would say stay away from pre-fabs especially due to your music interests. The majority of these boxes are either vented and tuned to 50 something hz or ported and tuned to 35 or so hz. They're also made of 1/2" or 5/8" MDF and aren't sealed very well along the edges. It's good for getting into the hobby for cheap, but your music preference just isn't conducive to cheap pre-fabs unless they're sealed, at which point you'll lose a ton of efficiency. They're more geared towards people who want to shake their mirrors for cheap, not accurately repeat kick drums and bass guitar lines as if it's live.
Bob's recommendations include a good brand named Taramps which are known for cheap but high quality amps. I recommend them as well, but something to be aware of with many of their models (including the multi-channel one mentioned, TS400x4 or DS-440x4 can't tell which was intended), is they don't have a variable highpass/lowpass cutoff so you'll be reliant on either a sound processor bought separately, the head unit's limited fixed HPF options, or the amp's limited fixed option for picking your crossover points. They're most well known for their more powerful models like the MD1800 mentioned which does have a variable crossover points.