Ebay for vintage - reputable sellers list?

The best way to determine the "safety level" of a seller is review all feedback, items sold and items bought.
Also check ID history, you get to recognise old IDs that rang a bell in the past.
Look for weasel words in descriptions and study photographs over and over.

And then there are the unknowns.
A very low feedback seller who has only bought shoes or CDs but is selling a vintage watch, usually accompanied by amateur photography and a generic description.
These are sometimes children or nephews passing on something they have no appreciation for.
These are worth further investigation and even communication with the seller. One benefit is that if the item is priced too high and it's passed in they may be open to offers and you have already established a rapport with them.

And finally, do your last minute check here (but not at the last minute).
And don't be scared that you will expose your secret treasure, you'd be naive to imagine that hundreds of other collectors (here and elsewhere) don't have multiple auction house, eBay, Gumtree etc searches set up and may have passed on your target for various reasons that they would be willing to divulge.

And to quote one of the real professionals in this game:

Good Hunting!

You may already know this, but this Forum is a great source...

I've found most of my vintage watches right here, with about 95% of those from private owners.

The nicest thing is, you can be almost 100% certain of your seller to be honest, forthright, and accurate!

I bought 2 watches from elady in 2014. No issues