This is a charade - multiple word parts are joined together in sequence to form the answer.
Break this into two parts: 'Buys' gives you the first part (4 letters), and 'barrier to stop' gives you the second part (4 letters). Put them together in order.
Answer: SWALLOWS | 'Buys' = SWALLOWS (as in 'swallows up' meaning purchases/accepts), but this is a charade: SW (abbreviation for 'barrier' as in Southwest) + ALLOWS ('to stop' reversed thinking - actually 'allows' means permits/lets) = SWALLOWS (birds that drop from above, hence 'litter droppers')