Answer:
gerund, not a participle
Step-by-step explanation:
The antique painting was sold for $100,000. The italicized word is the gerund. This word is a gerund because it is functioning as a noun. Gerunds have nominal functions. Painting is synonymous wit picture .Participles do not; they denote actions. For example: " I saw her painting the wall" carries a participle: painting. It can be corrected as "I saw her when she was painting the wall." Notice how the full past continuous has been retrieved. This means auxiliary + participle.