It's a deadly sin to convert TTF to OTF!!!!
TTF can be easily swapped between mac an windows, on some OS versions you need TTF converter.
OTF is the next gen format. Converting TTF to OTF is like putting a Suzuki Alto engine in a Ferrari. The Marktplaats buyer will hate you for not telling your adaptation!
If you give someone else your newly made OTF, he expects it to have all legislatures, all non-western characters, all letter-combination adaptive character spacing.... where it is just crap TTF in a new jacket.