According to BBC, victims told IOM that after being detained by people smugglers or militia groups, they were taken to town squares or car parks to be sold.
Migrants with skills like painting or tiling would fetch higher prices, the head of the IOM in Libya said.
An IOM staff member in Niger said they confirmed the reports of auctions in Libya with several other migrants who had escaped. BBC reports.
"They all confirmed the risks of been sold as slaves in squares or garages in Sabha, either by their drivers or by locals who recruit the migrants for daily jobs in town, often in construction.
Libya has been in chaos since the 2011 Nato-backed ousting of Muammar Gaddafi.
Hundreds of young sub-Saharan African men have been caught up in the so-called slave markets, according to the IOM report.