Before any number, one question: is this an original, a limited edition print, an open edition, or a reproduction? Those are four different markets.
What moves the number
Which of the four it is.
The artist, and what can actually be supported about the attribution.
Medium and size — originals above prints, and larger generally above smaller until delivery becomes the problem.
Condition — surface, paper, and whether anything has been restored.
The frame, which is a separate object with its own value and its own delivery risk.
Why two of the same differ
One listing shows the back — labels, stamps, old repairs, a relined canvas. The other shows the front in a frame. Everything that has happened to a work is recorded behind it. Describing artwork condition.
How to find out rather than guess
For an attributed work, compare completed sales by the same artist in the same medium and size, and discount for anything you cannot support.
For an unattributed work, you are pricing decoration: size, medium, quality of execution, condition. That is a real market and an honest one.
For your own work, the floor is materials plus hours. Pricing handmade work sets out the method.
Signatures and attributions
Nobody authenticates a work over a video call. An unsupported attribution does not raise the price in a room that knows the field — it lowers the credibility of the rest of the listing.
Questions people ask
Is an unsigned work worth less?
Usually, not always. Say what else is known about it.
Does a frame add value?
A little, and a lot of delivery risk. Price the work.
What is an artist's proof?
A print outside the numbered edition. Ask or say how many there were.
More: pricing art, selling art.