The platform to best reach the target market is Facebook. That is also the social media most heavily used by the competition. Only seeing competitor’s ads will make the competitor more salient in the customer’s mind.
For the first advertisements, I would want the message to reflect the season of spring. I always see the phrase of ‘spring cleaning’ and that would be cohesive with taking clothes to a consignment store. Focusing on getting new inventory would have the store stocked as people come in to consign their clothes.
Another set of ads could feature some of the best clothing received and have a message along the lines of “new arrivals!” This could entice shoppers to come in and find their own unique piece of clothing.
I personally see their competitors’ ads around once a week, so I think that would be a good frequency to post ads.
I believe the client would be able to budget for an ad. This would be much cheaper than creating a whole new website (they do not have one). Having ads on Facebook would help generate likes more quickly.