Just a warning, a lot of Office 2013 keys that you will likely be sold will be from MSDN subscriptions. These keys can be revoked at every time if the admin account they were generated by is reported. However, I bought a key from an MSDN subscription a while ago from a seller here and it is still working fine.
If you want a legitimate fully working key, it will likely cost you quite a bit of coin. On the other hand MSDN subscriptions are substantially cheaper however come with the risk of being deactivated at any time.
Good luck with your sales.
Thanks for the word of warning. I've bought a key but I will be on the lookout.