I wasn't going to post in this but...
Find a company(s) you trust and order with them and let them worry about inventory while you go wheeling. There are a lot of small companies on this forum that work hard to have happy customers and that is reflected in some of the post you see about them.
It’s true, that there are virtually no system to system real time interfaces, they just don’t exist in this business (yet). It's also true that a heck of a lot of companies drop ship, from EAD to Amazon and between. A lot more of this stuff comes from distribution type companies that don't sell to the public than you might think. Some big parts distribution centers have interfaces for vendors to check inventory but it's not a programming type of API. In some cases all we have is a weekly spreadsheet with quantity on hand that we import and update the database with. It’s not perfect, there are holes, but it is what it is right now.
OMIX/Rugged is a good example actually, few companies stock their products and you (the consumer) can't buy from them direct. It would make no sense for me to stock much of their line when they have everything in massive distribution centers on both coasts. We are talking about over 16,000 products that are currently in-stock.
The other end of this is the fab shops. We sell bumpers for example from a few mid sized companies. Again, it doesn't make much sense for us or anyone else for that matter, to have them in stock. Especially the slow sellers. I think I've sold 2 CJ bumpers in the last 12 months, I'll let those guys worry about keeping track of those.
In my experience, it works without a too much of a hitch. Sure my inventory numbers are not 100% because of the weekly import and in some cases non existent. In all cases I rely on my partners (Synergy, Spyder, GenRight, OMIX and many more) to have inventory and for the most part they do, they want our business, kinda helps them to pay their bills
Sometimes I can't fulfill an order. The key I’ve found, is to tell the customer as fast as you can and give them their options. That's really what it boils down to.
I needed a wheel for my lawnmower that apparently I bought over 15 years ago. After two trips to two stores I said screw it, googled, found one online and ordered on a Friday afternoon. I got an email Monday saying there was 5 day delay and I could wait or cancel. It shipped the following Monday and I received tracking. I thought that was a good on-line experience because of the email on Monday, the first business day after I placed my order.
I don't mean to offend anyone just trying to post from another side.