validate commerce order pre-placement
I created a CheckoutPane plugin which worked well in order-information step as it can can include form fields so can use form-validate function to stop progress until cart manually corrected.
In theory the stock could change between order-information and review steps. The panel is automatically processed in review step but it is only a display string not a form so cannot enforce anything here.

It is possible to use  HOOK_form_FORM_ID_alter (mymodule_form_commerce_checkout_flow_multistep_default_alter) which is called on submit of review step - so can add a validate function and stop things

The Pane approach gives a better UI space for explaining things to user as HOOK.. can mostly only generate a validation error.