It uses data like any video, which is to say a lot over an hour. There is no special low-data mode, so the saving comes from how you watch.
The four things that work
1. Join for the item, not the hour. The single biggest saving. Most shows list what is coming up, so you can arrive for the thing you want and leave after it.
2. Set a reminder for the show instead of waiting in it. Waiting forty minutes for an item is forty minutes of video you did not need.
3. Close the tab when you stop watching. A show left open in the background keeps streaming.
4. Lower the quality if the player offers it. Smaller picture, much less data — and the auction is unaffected.
You can bid on almost nothing
Worth knowing, because people assume a weak connection means they cannot take part.
Bidding is tiny — a few bytes to the server, nothing like the video coming back. So if you are rationing, you can watch briefly, leave the show, and still bid.
And if you drop out entirely, your bid stands. Leaving does not withdraw it. If I leave the show, do I lose my bid?
What does not save data
Muting it. The audio is a tiny fraction of a video stream.
A smaller phone screen.
Turning off the chat, which is text.
If the connection is the problem rather than the allowance
Different question. Steadiness matters more than speed — a stable 2 Mbps watches a show and an unstable 20 stutters. Mobile data is often steadier than contested wifi. How fast does my internet need to be?
Planning around it
Follow the stores you buy from and let the notification bring you at the right moment, rather than browsing live shows to see what is on. How do I find a seller I bought from before?
Browse listings instead of shows when you are rationing. Ordinary listings are pages, not video, and a lot of what sells here never goes into a show at all. Buy now, explained
More: what you need to watch a show, how fast does my internet need to be, can I watch a show after it has ended.