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You'd spend more time and effort trying to sell it than anyone would be willing to pay, really.

Selling an IPB 1.3 theme is near impossible. Some of the best forum designs this world has seen were on invisionfree.com and the ifskinzone, all themes were free and posted by hundreds upon hundreds of designers. I am proud to have been around those days, some of the work was unreal.

Not to mention that invisionfree is now done with IPB 1.3 and is now using Zetaboards.