1. Inventory systems very rarely live in isolation. If you're a medium/large business, you quickly discover the need for integration with accounting, procurement and manufacturing or production. If you're a small business you may only need accounting -- but nevertheless selling this by itself might be difficult.
2. You're using strange terminology that unfamiliar to those dealing with inventory day-to-day. What is a 'sheet'? Why is more sheets better than less when it comes to pricing? Fix your terminology or at the very least clarify what it means.
3. You're charging too little. You're competing with a bunch of companies that won't even list prices on their websites (which means it's in the 5+ digit range per month). Your dirt cheap pricing gives the impression that you're a tiny app that has little value. I'd suggest you consider pricing based on number of users, similar to Atlassian.
4. Sorry to be blunt, but your website sucks. You need more screenshots. Pricing should be on the front page. There's an overwhelming amount of text on your front page, as a customer I probably won't read all that so don't bother.
5. Stop emphasizing that you're a one-man team. That is not attractive to business customers. They want to know that there is firepower behind the product and it's not a fly-by-night operation, both to handle issues when they come up as well as grow the product down the road.
Your big advantage here is transparency. Be up front about your pricing and your features, this sets you apart from your competition by leaps and bounds. Your competition requires a phone call with a sleazy salesperson and then they price based on your revenue. You're in a good market, but it will require you to be a lot larger than you think. You're actually competing in the ERP space. That's actually a good thing though, because nobody likes working with those companies, they're horrible. Use that to your advantage and sell your customers on your advantages.