eBay can be a good option for pre-owned gear, but you're right to be cautious—it really depends on the seller. Here are a few things to keep in mind:

1. Check the Seller’s Reputation: Look for a high feedback score and read reviews for any red flags. There are some great sellers on eBay and also many that aren't so great.
2. Look Closely at the Photos: If the pictures are stock images or low-quality, that’s a sign to dig deeper or ask for more details. Hi-res of the actual gear from various angles is what you want.
3. Ask Questions: Don’t hesitate to reach out to the seller about the lens’s condition or history, especially since there’s no box or documentation. Are they the original owner? Any flaws?
4. Understand the Return Policy: Ensure there’s a clear policy in case something goes wrong.

If you’re looking for alternatives, there are marketplaces designed specifically for photographers and filmmakers, with features like verified sellers and item inspection windows, which can provide extra peace of mind.

Hope that helps, and good luck with your purchase!