By itself, buffer stock can either be both brand new or refurbished. All depends on the warranty provider.
In any case, rma buffer stock simply means the upfront stock that the provider has at its premises to be able to replace customers' faulty item one to one.
No buffer stock will mean that the provider will need to request stock from the distributor/manufacturer, which will take sometime.
As an end-user, u just need to know that no buffer stock means u will not be able to get replacement one to one, or will have to settle for buffer stock of an alternative model (eg: if gfx card is faulty and no buffer stock, that means may have to temporarily exchange for other gfx model until new stock arrives).