I woulden't sell anything in big style on Turbosqid...they take 50% But It is a good possibility to promote your products by selling either just a few products of your whole product-range, or offering some free sample downloads. I did that to promote my plants in the begining...but now sales work much better from my site or from Gamebeep (Bret the Gamebeep guy is cool and takes only 20-25%).

Sorry...back on topic: The marine cannon looks pretty good but has indeed a rather high polycount for it's shape...reduce it a bit, make a whole marine arsenal, create a website and offer your content for free or sell it for a reasonable price