The scheme is economically efficient; trading occurs between different sectors: those which can make emission reductions cheaply and those which can’t. The shipping sector would be the net buyer of emission allowances/credits from other sectors for the foreseeable future. These purchases would cover the reductions they cannot make cost-effectively themselves and would be purchased centrally from the funds aggregated in the scheme (purchased from the power sector, REDD, CDM/JI, etc.). The shipping industry will also have an increased incentive to become more fuel-efficient, as the total levy paid is directly linked to the amount of fuel used. The trading aspects of IMERS, its links to emissions trading schemes have been independently analyzed and endorsed.
The incentive for selling unused allowances depends on the cost of their acquisition, and may not exist in reality. The only cap-and-trade scheme with relevant details in this area is METS, introduced by Germany for furthering discussions on the topic. In METS, emission allowances are auctioned and none are allocated for free (see IMO GHG-WG 1/5/7). Therefore the incentive from selling unused allowances does not exist as the price of selling would equal the acquisition price (either at auction or from the carbon market).