I thought AirBnB type rentals were illegal too. But they are readily available! Having rented condos before in BKK (long term with a proper annual lease) I would be wary of AirBnB. Thai furnishings, even in upmarket places, will normally mean a rock hard bed, so hard that many westerners will struggle to sleep well. Many condo renters have to arrange their own mattress or a topper. And these problems are secondary to the fact that under Thai law a short term rental is illegal.