I bought a 2nd hand gold plated R&C saxello a few years ago from eBay and I've bought quite a few other horns by that route. I have been incredibly lucky with the state of these horns basically bought blind.
The R&C is a good horn with fine tuning and pretty comfortable. The mouthpiece that came with it wasn't great, but the case is very nice and the price was OK. The action is a bit "spongy", not nearly as crisp as my Yanagisawa, but it is OK.
If you have a chance to try the horn before you hand over the cash, then here are some things to look out for:
1. Bent body (not always easy to spot, but surprisingly common and quite expensive to sort out);
2. Dents and dings - have a very good look, especially near toneholes where they might be hard to sort out;
3. Seized or sticking action - could be minor causes like a spring out of position or more significant things like corroded or bent rods and mechanism;
4. Noisy action - check for metallic clattering, probably due to missing felts or corks;
5. Bad repairs - from poor solder finishing to fitting parts from other horns that don't fit properly;
6. Damaged finish - should be pretty easy to spot, but think how much it matters to you;
7. Leaking pads - can be hard to work out how many would need to be replaced or reset and how much that might cost. If it's hardly been played, then this shouldn't be a big problem.
Overall, take a good look and then take your time playing the horn, if possible with a mouthpiece and reed that you know and are comfortable with. Playing with a mouthpiece that is new to you could make you reject a good horn.
For any (unadvertised) problems you spot, think how much they matter to you and how much it will cost to sort out, then try to negotiate the price down by that much.
Good luck
Rhys