My highest preference is KEH and Samys. I get to see and handle the equipment at Samys and I get the honor of paying state sales tax in lieu of shipping. There is the instant gratification factor and they are competitive or better than KEH prices. I have done well at KEH. Only one problem and that was resolved by them sending the replacement before I shipped back the original item.
Close after KEH is B&H. I have only made a few purchases on APUG Classifieds and they have been good experiences, too.
I have done well with eBay stores including KEH. I look for well known stores with good reputations. As always with any internet purchase look at the photos, read the details and ASK questions.
I have had good experiences with individual eBay sellers by looking at ratings at or near 100. I tend to look at the sellers with the higher sales numbers. Every once and a while I find something that a seller with less than 20 sales with exactly what I am looking for. With those again I have had good luck and occasionally they will throw in some extras because I took a chance with them.
My biggest problems have been caused by GAS pangs and the desire to get one more lens or accessory. I have found no solution to this problem and now I accept it as the price for the interest in photography.
My experiences YMMV