Customers in brick and mortar furniture stores, unlike in online furniture stores, do not impulsively buy things besides what they came in for, not purchasing a bar stool and bookshelf when they come in for a couch, but they leave with a couch only.
(A) in online furniture stores, do not impulsively buy things besides what they came in for, not purchasing a bar stool and bookshelf when they come in for a couch, but they leave with a couch only
-- a.1"Customers in brick and mortar furniture stores," is compared with "Online furniture store"
-- a.2 before "but" we have verb-ing modifier and after "but" , we have a clause. So Issue with Parallelism.
(B) those in online furniture stores do not impulsively buy things besides what they came in for; do not purchase a bar stool and bookshelf when they come in for a couch, but leave with only a couch
-- a1 is fixed.
-- b1- the sentence after (;) should have one subject do not. The structure looks awkward. Let's look for a better choice.
(C) online furniture stores, do not impulsively buy things besides what they came in for; someone will not purchase a bar stool and bookshelf when they come in for a couch but will leave with only a couch
-- a2 is fixed but a1 is still there.
(D) those in online furniture stores, do not impulsively buy things besides what they came in for; someone who comes in for a couch will leave with a couch only and not purchase a bar stool and bookshelf as well
-- This is the correct choice.
(E) in online furniture stores, do not impulsively buy things besides what they came in for, not purchasing a bar stool and bookshelf when they come in for a couch, but they leave only with a couch
-- a.1 and a.2 issues.