Art sells on two facts — what kind of object it is, and who made it — and most mistakes blur one into the other.
Not saying original, print or reproduction
Four different markets. All legitimate to sell; only one of them is a problem, and that is a print sold as an original.
Attributing confidently
"Signed, I cannot identify the signature" is a good sentence. A name you cannot support is the thing that unravels, and it takes the rest of the listing with it.
Blending attribution into condition
Who made it belongs in its own sentence with how you know. Describing artwork.
Framing before selling
Glass in a parcel is a risk to the thing it protects, and the buyer would usually rather choose the frame.
Giving one measurement
Image size and overall size, both, in centimetres. This is the category where scale surprises people most.
Photographing under warm light
Colour under a lamp is not colour. Daylight, no glare, and a second shot at an angle so the surface reads.
Plastic against the paint
Glassine or tissue first. Plastic directly on a painted surface traps moisture. Packing art.
The one that costs most
Not showing the back. Labels, stamps, relining, replaced stretchers, old repairs — everything that has happened to the work is recorded there.
Questions people ask
Can I sell a reproduction?
Yes, described as one, in the title.
Is an unsigned work harder to sell?
Not necessarily. Describe it well and let the room price it.
What about old tribal pieces?
A genuinely old piece may fall under antiquities rules. Ask first. Prohibited items.
More: selling art, what it is worth.