i.e. the garage sale took place in a highly-cultured, net-savvy area full of people who do bargain/antique shopping. Back in the late 2000s, my friends in the same area put a cheap 30 word ad in the local newspaper for a garage sale and got dozens of visitors. I don't know how much the ad cost but probably something like 50 to 60 dollars.
It's hard to gauge how much of a "success" this is when a Craigslist ad, plus some postings on large music mag forums (and I'm sure SF/Berkeley have their own large local websites) would've drawn the same attention...with a little more handiwork on the seller's end, but without costing $150.
* edit: I'd even argue that he might have drawn more people in with Craigslist and a few other nichey-type posts. I'm not a record-collector myself, but I'm guessing that there's a fair amount of them who are used to "digging" to find gems, as opposed to checking out ads/events on Facebook...antique collectors regularly scour the old-fashioned newspaper ads for garage sales because they know the type of old-fashioned people who might have undiscovered antiques are also the type to advertise in newspapers.