IIRC .380ACP/9x17 and 9x18 mags for Maks are basically identical and completely interchangeable.
Re: where to sell it, there's always the TFL forum classified ads or Gunbroker. If the magazine is a standard single-stack or "post-ban" 10rd double-stack, it can be listed on Fleabay. Only the 12rd "pre-ban" double-stack mags are over the eBay 10rd limit; these are the rarest and most valuable Mak mags, as the importers do not seem to have sold many spares.