Haven't really had any bad vendors - do your research BEFORE HAND is key. Anyhow, here are some of the vendors I used:
EVoffer: I got power frunk from them, lots of documentations on its site, had some issues w/ installing and I was able to get support quickly to resolve.
Maxpider 3D mats is good. Good fitting overall.
Snapplate: I had a small issue w/ snapplate fitting and the owner was very responsive and had it resolved right away.
Enhauto: I got S3XY buttons for $201.99 on sale during Christmas - good deal and seems to be the best price overall anywhere for S3XY buttons
TWRAPS: good place for decals, small wraps, etc.
Tesla: charging accessories. Good overall.
And last but not least, Amazon, that's where I got the majority of stuff, travel boosters, 3M tapes (for spoiler below), Sun Shade, Camco charging cables, cleaning products, Tri-fold Twin Mattress, Jack pads, USB drive, etc. This is where research really comes into play.
Oh, I did get a V-style spoiler on eBay. It worked out wonderfully for me, but I wouldn't recommend it unless you know what you are getting.