The best for me so far is probably "Hollywood Vault" on eBay. I used to think it was Premiere Props, but they ave recently been inaccurate and have taken to substituting items won at auction.
Starwares has been accurate and friendly nearly every time.
The world-infamous Entertainment Movie Props a/k/a Puttieface has long earned a spot at the bottom of the heap. As near as I can tell, he would buy and sell authentic items and then every once in a while, he'd make up some fake tags and send out lookalike items.

Beyond that, he was notorious for abusing his customers with foul and quite colorful language.
But within the context of "reputable" sellers, Premiere Props has been making a lot of errors lately. They call them "inventory glitches." I buy a few hundred pieces a year from them, but when an eBay auction falls short of the average price for the similar item, you suddenly get a substitute item like a stunt outfit, or an over-teched version with a "sincere" apology.
Most recently, they didn't even pretend. I won an Owen Wilson shirt that was clearly screen worn with tags. The average auction price would be about $130 and a similar item would retail on their site for $500+ but the auction for this item ended at $17. So they just sent the rest of the items from my order and a note on the invoice that they had refunded the $17. Which they had not.
This is the 3rd order in a row that they have not sent the items won. It is also the 5th order in 6 months. Anyone else having this problem?