Shipping is part of it. Amazon generally doesn't ship to Alaska, and Prime doesn't change that. The same vendors on ebay usually will ship to me.
The bigger issue is that Amazon prices are usually higher on the stuff they will ship, and at best, Amazon has the same made-in-china junk that you find on ebay. Ebay sellers sometimes even have better products, like NOS parts.
Buying parts from Amazon is always a crapshoot, too. They do OK as a bookstore, but they're the worst parts store on the web. There are better book stores, too. I just ordered a couple of theology books from Abebooks. I got readable editions from 1848 for close to the same price as the nearly readable made-in-India editions Amazon was peddling - it's almost that NOS thing again. Abe, Alibris and Biblio are three places to shop for books after you've checked Amazon.
So, it's not that I won't use Amazon, but it's that I always look someplace else before I order anything, because I can often do better. Then there's the shipping problems, which are worst on Amazon.