eBay only removes items when somebody reports them
Plenty of bottles are sold without problems, but whoever reported your listing presumably considered that the oil paints presented a fire risk. Or even the whole case if it there were substantial residues of dried oil paint.
Whether they had a valid point is impossible to say without seeing the item, but such items may potentially present a fire risk. Have you checked the Post office regulations? These may not allow part used flammable paints or items soiled by paint residues to be carried. Dried linseed oil can be very hazardous, and anything contaminated with oil paint residues can fuel a fire.
Without more information it's impossible to advise, except to say that people who are not expert in this field don't always realise the hazards that items can present.
