What are the requirements for motorcycle trip across Europe with US registered motorcycle?

I hope someone better qualified than I will come to deal with the insurance and licensing aspects of this. The six words that worry me are "where I plan to sell it".

It is perfectly possible to import a motorbike to Turkey temporarily - that is, ride it in, then ride it out some (shortish) time later. This page discusses the process of so doing, but also notes

3. Vehicles must be taken out of Turkey at the end of the permitted import period.

and

9. The following actions are unlawful and subject to penalty:

—Not exporting the vehicle before the end of the legal term of importation

—Sale of the vehicle within Turkey

...

This article notes that

Your vehicle details will be appended in thickly scrawled ink to one of the pages of your passport at whichever border checkpoint you pitch up at, the date of entry will be stamped in and the important point to remember here is that the vehicle must leave Turkey whenever you do.

The same article also discusses importing the vehicle (ie, bringing it permanently into Turkey, which would enable its sale) and notes

you will find that the duty payable is a staggering sum (proportionate of course to the value of [your vehicle] here in Turkey).

So either you tell them you're going to sell it in Turkey and pay a huge sum in import duty, or you don't in which case you will find it difficult to fly home. I don't know whether you've looked at the resale prices for Harleys in Turkey and thought about a holiday partially funded by the sale of the bike, but if you have, now you know why resale prices are so high - the government wants a big cut, and has ways to make sure it gets it. My apologies if you already know all this stuff, in which case my answer is irrelevant and I will delete it.