Basic "button" and inventory capability in a shopping cart software

I'm sure it's been addressed (and I'm still digging through the archives) but this my current situation.

Existing website, sells 'one-off' products, I want the ability to have it shown as 'out of stock' as soon as somebody purchases it so it doesn't get 'double ordered' (so some sort of inventory integration)

Simple 'add to cart/but it now' buttons I can drop into the existing pages.

Everything I've run across in my initial stages of looking seems to be a 'all in one/website' type of solution instead of an add-in (or if it has options that simple, I'm too much of a maroon to find it )

Thanks tons in advance,