Stick with known brands, at least to start, unless the price is just insanely low (sometimes there are amazing deals...I picked up 15 un-stickered GFS 12kOhm Li'l Killers for $7 each last year, and a half-dozen very nice branded Wilkinson ceramic soapbars for the same amount the year before...almost the equal of a Kent Armstrong with a pair of a5 bars in it). Allan Eden is my favorite HK dealer, along with iknmusic, but I have gotten to know the proprietor at ikn. Allan Eden sells primarily Belcat, Artec (makers of GFS' pickups...check the bobbins on your GFS pickups!) and Wilkinson stuff, along with some of their own products. I bought a neck recently from a local hotrodder who got one of their circle-E necks...not bad, but no better than GFS. I've tried a TON of the Chinese hardware, bought a bunch more for a store where I do repairs and stock parts. It's hard to find a *bad* EB/MM-type bass bucker, but the no-name soapbars are no-name tone...the cheap p and j pickups are not fit for a partscaster. The Wilkinson ceramic soapbar isn't a bad pickup for $25; the Belcat "ceralnico" EB/MM has series wiring only, but a warm AND very powerful tone. The $25-30 Artec EB/MM is pretty nice for the $, and the blackout is identical to the open-pole in tone and resistance. The alnico p/j's that Allan Eden has - not sure if they're Belcat or Artec - sound IDENTICAL to the GFS vintage-p-bass, which IMO is the best under-$50 p-bass pickup out there.
For bridges, I've had spotty luck. Bezdez, a Canadian ebayer that handles primarily Korean stuff, has the best price/variety for bass bridges for me...standard p/j bridges vary little but their cast bridges (incl. unbranded Dillions, old pre-headless Steinberger designs, and even their cheap $40 piezo) have all been good for me.
Pegs are a different matter. I buy the Chinese pegs two sets at a time; they're not BADLY made, but often badly assembled...caps pop off from poorly-attached screws. Avoid the pegs with the thick Schaller-type keys which are junk; the thinner keys with the little crease in the center, had a few pop their covers but all were solid, reliable tuners.
When buying large-can pots, stick with the good stuff unless you do NOT like a free-spinning pot...the Chinese full-size pots are nearly all firm-feel, and require O-ring knobs for me to turn with my pinkie.
Necks and bodies, I would try to source domestically...it's mostly our wood anyway, and finish quality varies widely.
I've done probably 100 orders from Chinese vendors since 2011...I've had a couple lost-in-the-mail's and a couple of shorted orders, and maybe 1 of 20 pieces is defective or substandard, double or triple the rate of problems with stuff bought onshore...I consider that well worth the savings for the hassle and loss but if I was building a parts bass I'd consider reliable quality and fulfillment worth a 20-35% higher price. The only vendor with whom I've had problems with refunds and lost orders is jackmusical...I lived with it for a while since they had the best inventory/price profile in 2011; that's no longer true. My current favorite is Allan Eden, and their same-spec Artec stuff is considerably cheaper than the identical GFS-branded product, but there are several other vendors with comparable inventories who are likely as-good or better.
My usual recommendation to local kids wanting to build: source the mission-critical stuff (sometimes the neck, sometimes the body, sometimes the pickups) from a known-good source and be willing to pay extra; by all means tho go offshore for the stuff you consider less important. And generally speaking, I recommend hotrodding a quality instrument that isn't quite where you want it over building a parts guitar...it'll be a LOT easier to get a decent dollar down the road, esp. if you save the original parts, and you'll usually get more guitar for your dollar when you're done than you will from a scratch build...but of course there are always situations where you simply can't buy anything close to what you want to build.
Hope that helps.
