You can get lucky. One beginner of my acquaintance asked a local jazz bass star for advice where to buy a cheap beginner's double bass, and the guy immediately said he'd be prepared to sell his own first-ever instrument, since he'd barely played it for many years. Plywood, not much to look at, but well set up and with an acceptable sound. He wanted 300 EUR for the bass, including an Underwood pickup, a spare set of Thomastik strings and a 90 minute first lesson. My acquaintance tried to persaude him to take more, but he just wouldn't hear of it.
That's a one-off, but I think there are bargains to be had in eBay, as long as it's close enough to go and see it before you bid and you can take somebody with who knows about basses. Don't be put off if the bass needs a few hundred Euro spent on it for setup, new strings, and minor repairs - that's normal.
Alternatively, you could look into renting. When I was wavering about whether or not to try my hand at double bass, what settled the issue was the discovery that I could rent one from my local music store for only 35 EUR per month, minimum rental 6 months. It was a plywood bass from Gewa, worth 1450 EUR new, not one I'd want to own in restrospect, but well set up and perfectly OK for a beginner. The rental cost was deductable from the purchase price if I subsequenly bought the instrument, or indeed anything else from the store, so you couldn't really go wrong with that deal.
