I've bought plants from Brent at Evergreen Gardenworks. Great service, great shipping, never a problem. Highly recommend. I've also bought from Meehans and same thing, excellent service.
Have bought a few times from sellers on ebay and never had a problem. One time a plant was shipped to me in a somewhat flimsy box and not packed well but it survived and is doing fine. I always look at the feedback for ebay sellers, if there are more than 1 or 2 (depending on how much they sell) recent negative reviews I take that as a bad sign (but always read what the complaints are, sometimes it's obvious the buyer is the problem). And I'll send questions to the seller if anything is unclear, like what species the plant is, etc. The type of response is indicative of what kind of seller you're dealing with.
Have bought and sold on the facebook auction sites (mostly 99 cent bonsai). No problems with trees (I did have one non-paying bidder but he was dealt with). Have had issues with pots, two sellers shipped me pots with inadequate packaging and the pots arrived in pieces. They both refunded the cost without any argument, but I doubt I'll buy from them again. In contrast, I bought a couple of pots from Bonsai Mirai and they came so well packaged that you could possibly drive a truck over them and they wouldn't have been damaged.
Wire I get from Julian Adams (copper) or Stone Lantern (aluminum). Great service from both.
Edit to add - have also bought things from ForestFarm.com and Matt Ouwinga (kaedebonsai-en.com) and recommend both.