Beside the fact that it is certainly a bit early to call it quit, I'm having problems with the product and the site itself.
I have dealt with stock inventory for years and it is one of these areas that looks deceptively easy to implement but ends up being a lot more complex in real life.

The website makes many grand promises but there is very little information on what it actually does.
The target customer should be someone knowledgeable in Stock Inventory, but there are not many specifics for them to even assess if the product is for them or not.

* Who does this target? small businesses? retail? factories?

* What is a "sheet" really ? It is a Form? a data entry grid?

* Inventory data doesn't exist in a vacuum, it needs to be fed to accounting software, so at least a good import/export/reporting story is important here.

* similarly, can I connect to the database to produce my own reports and plug custom software (even Excel for instance through a datasource) to it?

* I can't find any information about cost management. Does the software only manage quantities or can it track cost as well?

* How does the software manage counting units? Can I manage cable lengths in metre or in whole roll? Can I sell some oil SKU by the litre and other by the bottle?

* What's the story for annual/continuous inventories (stock counts)? does it help me with that?

* Cant I easily make transfers from one warehouse to another?

* I can't find whether the software is able to manage FIFO, LIFO, batches, etc.

* What's the picking list story if there is one?

* Does the software manage part revisions? For instance factories may have different revisions of the same item (electronic or mechanical).

* Does the software manage stock locations so I can easily find where product with SKU F7786 is located?

* what's the story for receiving deliveries? Can I have a quarantine area where I receive my goods for inspection?

It reads everywhere that this is a one-man labour of love.
Why should I risk an important business activity to a one-man shop? What happens if the founder is sick and there's a problem?

I don't want to be too negative, I'm only being harsh because I really like the concept; the design and the amount of work put into this is a short time is amazing. The tablet story is great too.

On the other hand, it feels like this was made by a programmer who doesn't really know the business he's in and ends up with a products that seems to be targeting libraries and bookstores (2 very different businesses) and groceries, retailers, even factories maybe.
It tries to be too much for too many.

To save this, I would target specific markets and make appropriate versions of the software for those markets, using their business terminology, highlighting on the first page the pain points they are facing and how the software solves them, etc.

Making a one product fits all is exactly the sort of problem small business face when they buy a cheap general purpose ERP: it's often too complex and it ends up not being what they need.
The pricing is also fairly high: a 4-seat version of Sage Line 100 (a complete ERP for SMB) costs less than US$900 per year in license renewal. Yours cost about $700 for 3 users only for inventory management. That's quite a lot.