From the Turbo Lister FAQ:
I can’t find the Best Offer option, where is it?
The Best Offer option will appear in the "Selling Format" section of the Create/Edit item pages. If the option does not appear, that would indicate that one of the options required to support Best Offer have not been selected. Choose your format (Fixed or Store Inventory format support Best Offer) then choose your category. If your category supports Best Offer, the option will appear.