@zavato. I think, if one has taken the LAS's time to audition its item and it was cool about it, one should then give the LAS to opportunity to match that online 1/2 price before clicking "buy-it." If it's close, probably better to go with the LAS, particularly if it has a good reputation for questions and support.
"Obligated" to pay double? No. Morally obligated to give the LAS a fair shot to earn the business by price or service? I personally think so.
I suspect some places, if you are up front about it and they aren't busy, will still offer a brief audition even if only to try to establish a relationship or future sale, even if they can't/won't match the online price. Or offer some added benefit--maybe delivery and set-up or something to justify a slightly higher price.