At least in the chemical sense, the term “buffered aspirin” is used incorrectly.
Aspirin is a weak acid. In aqueous solution, it ionizes according to the equation
HA + H₂O ⇌ H₃O⁺ + A⁻
The stomach contents are highly acidic, with a between 1.5 and 3.5.
This pushes the position of equilibrium to the left, so the aspirin is in its covalent HA (fat-soluble) form.
It easily diffuses through the stomach lining and into the cells underneath.
Once there, it encounters a more basic pH.
It ionizes again, and this prevents it from diffusing back into the stomach.
Aspirin suppresses the production of hormones that stimulate blood clotting, and this causes capillaries in the stomach lining to leak.
For some people, this can result in serious blood loss.
Keeping the aspirin in its ionic form until it reaches the small intestine would prevent it from causing bleeding.
Some manufacturers add “buffering” ingredients such as MgO or CaCO₃ to the aspirin to neutralize the stomach acid and raise the pH of the stomach contents.
But the reactions are just strong acid-strong base reactions.
MgO + 2HCl → MgCl₂ + H₂O CaCO₃ + 2HCl → CaCO₃ + CO₂ + 2H₂O
They are not buffers in the chemical sense.