we sold on facebook. join loads of the bangkok group and grung thep (using thai script) and write a nice ad with good photos, then post this to all the groups you have joined, then bump up the listings every week or two
start at the price you want, then slowly drop the price if you want to. give it lots of sales spiel to make it sound like the bargain of the year
also, post physical for sale signs in reception etc of teh condo building
tell friends and offer them 3% / 5% of the sale if it completes with their buyer