My (expensive) experience from eBay is that anyone advertising their items as 501st approved aren't. Guys who make great stuff don't advertise that way. Christian on Man of War Studios, for example, sells stuff on eBay and nowhere do his ads say "501st Approved" or "501st Approvable." Etsy might be a little different. I have seen some good looking kit on Etsy, but I wouldn't buy from anyone not on this site unless there was a recommendation from someone on this site.
The vendors who advertise in Cargo Hold here sell top quality stuff. It will take a while to get everything together. Take the time to research before you buy a thing. I bought some stuff that looked right to me... it turns out, I didn't have a detailed enough eye to be buying anything. People were here giving advice to do this and I didn't listen, or at least I thought I knew enough. But I didn't.
I thought I was going to save time buying something already painted. It turns out that what I didn't have to throw away, I had to strip the paint off and start over. Which is extra time and my starting point was not as good as if I had bought an unfinished product.