Two questions decide most of what you need to know, and neither is about whether you like it.
"Is it an original or a print?"
And if a print: limited or open edition, numbered or not, and printed when. These are different objects at very different prices, and a good seller answers in one sentence.
If the seller does not know, that is an answer too — bid as though it is the cheaper of the two.
"Can you show me the back?"
Labels, stamps, inscriptions, old gallery marks, repairs, replaced stretchers, a canvas relined. Almost everything that has happened to a work is recorded there and nothing of it faces the camera.
Then
- Measurements, image and overall, in centimetres
- Materials and support — what it is painted on, and whether it is stretched, mounted or framed
- The surface at an angle, for texture, craquelure, flaking and restoration
- In daylight, because colour under a show lamp is not colour
- Signature and edition number, close
On attribution
Nobody authenticates a work over a video call. A seller can show you a signature, a label and a history; none of those is an authentication. If the name is the reason you are bidding, weigh the price against what has actually been shown, and treat an unsupported attribution as unverified.
Framed work
Ask whether it will be posted framed. Glass in a parcel is a risk, and collection is often the better answer for anything large.
What protection covers
A print sold as an original, undisclosed restoration, measurements that are materially wrong, or damage not shown: not as described, seven days from delivery.
An attribution you accepted as unverified is not.
Questions people ask
Is an unsigned work worth less?
Usually, and not always. Ask what else is known about it.
What is an artist's proof?
A print outside the numbered edition, usually marked AP or similar. Ask how many there were.
Should I buy unframed?
Often yes. It travels better and you choose the frame.