For eBay, the important thing is to "buy" the seller, so check out his credentials and satsifaction points. There are a lot of fakes on eBay.
Buying at Bonhams (the others auctions houses that sell watches in Europe are Antiquorum and Patrizzi) is easier than you think, but I do recommend viewing the watches first hand before bidding on anything.
For Chrono24, I'd only buy from someone who is identified as a "Trusted reseller".
There are many direct dealers who sell second-hand, and watches.co.uk is just one that I can recommend. I particularly like the way they rate their watches on a scale of 10, and if you buy one rated 10/10 (which they mostly have), you'll get a watch in pristine and almost new condition.
Some final points:
- second-hand and vintage watches may need to be serviced, so don't always expect these watches to keep perfect time! I would recommend budgeting the £200/EUR 240 -£300/EUR 360 for service in your calculations.
- Vintage watches usually have plexiglass which can scratch easily, they also might not be waterproof, and the lume might be worn off (hands and numbers won't be visible in the dark).