Ask before the lot runs. On a standard lot there is room near the end; on a sudden-death lot the clock never moves.
The questions
"Is it an original, a limited edition print, an open edition, or a reproduction?" Four different markets. All legitimate to sell; only a print sold as an original is a problem. Non-answer: "It's an artwork."
"Can you show the back?" Labels, stamps, inscriptions, repairs, a relined canvas. Almost everything that has happened to a work is recorded there.
"What are the measurements — image size and overall?" Both, in centimetres. Scale surprises people most in this category.
"What is it painted on, and is it stretched, mounted or framed?" It changes how it hangs and how it travels.
"Has anything been restored or retouched?" Declared, it sells. Undeclared, it does not stay sold.
"What do you know about the artist?" If the name is the reason you are bidding, ask how they know. Nobody authenticates a work over a video call, so an unsupported attribution should cost the seller your higher bid rather than earn it.
What protection covers
A print sold as an original, undisclosed restoration, measurements materially wrong: seven days from delivery. An attribution you accepted as unverified is not.
Questions people ask
What is an artist's proof?
A print outside the numbered edition. Ask how many there were.
Should I ask for it unframed?
Often yes — glass is a risk in transit and you may prefer your own frame.
Can I ask to see it in daylight?
Yes, and you should. Colour under a show lamp is not colour.
More: buying art, what it is worth.