I don’t have experience with the Silvia but I do with the GCP and I think most of what I used when buying my GCP should carry over for the Silvia.

Figure out what serial or model number you want. I know for GCP there’s a list which breaks down changes. I found which R/SIN number I wanted for mine, I’m not sure what Silvia’s are but I’m sure there’s lots of resources out there.

See if there’s any common issues with that particular model. Maybe it has a leaky steam wand, maybe the boiler has a common fault, look out for those when shopping. Also keep in mind how likely the issue is to resurface and how difficult it is to fix. It may be worth saving a bit of cash and getting one with the issue if you can fix it yourself relatively cheaply and easily. Or maybe it’s such a hassle or expensive for parts that it’s worth to spend a bit more for one in good shape.

Generally, I’d say the shape of the boiler is one of the bigger tells for how good of shape the machine in general is likely to be. If the boiler is in excellent shape, good chances the owner takes care of it and you might not have to worry about it as much.

Ultimately, the job of an espresso machine is pretty simple. It needs to heat water and push it through a puck of grounds at high pressure, that’s about it. The GCP is pretty simple internally and I’m comfortable taking it apart and inspecting it if necessary so I was comfortable getting a cheaper one. I happened to luck out and got one in great shape, but that’s probably not the normal condition for the price I paid. I’d imagine the Silvia would be similar.

Finally, if you can, pick up the machine in person and ask them to pull you a shot first and show you how it works. This is mostly to make sure it’s running properly and see firsthand if there’s any issues.

Hope this helps a bit.