From the dealings I've had with them I understand they are effectively a drop-ship organisation.
We order stuff believing them to be in stock, then they arrange their suppliers to ship direct, which is why when you order several things they can arrive in different packages at different times.
If their supplier is out of stock or doesn't dispatch quickly we improve their cashflow for a few additional days on top of their normal terms.
Assuming of course their supplier is still shipping against their account because they are up to date with their invoices
Done well it works effectively, but takes good supplier agreements and exceptional organisation, none of which would appear to apply to MMS.
The Marine Superstore on the other hand are excellent.