Purchasing Pre-owned?

[Moverman1]

New to the forum and came here for advice from the experts. I'm looking to buy a pre-owned datejust. 36mm, steel/18K fluted bezel, jubelle band, with preferable white dial. It's important the watch is authentic in and out since it will be an appreciating heirloom. After researching the best solution seems to be a private owner sale/purchase but that's unlikely.
The online stores are subject. I could go that route then authenticate with a jeweler but reviews say it's tough to return. Bob's Watches looks to have the only and best authenticity guarentee. Any suggestions are appreciated including biting the bullet and buying new.