To summarise:
a. When you post an ad, you are giving an "invitation to treat", ie inviting people to make offers for your item at the specified price.
b. When someone makes an offer at the price given, you can then tell the prospective buyer whether you accept his offer.
c. If you accept his offer, a contract is then created, because (i) he has made an offer; and (ii) you have accepted the offer.
Thus, if you do not go on to fulfil the contract, you are in breach of the law of contract and can thus be sued for damages.
(d) It is also possible that the offer made is a conditional one (ie not binding until the conditions are satisfied), eg. subject to satisfactory viewing, subject to producing the item for sale by a certain date/time, etc.
If so, then the buyer need not fulfil the contract if the conditions are not met reasonably. There is no breach of contract in such an instance.
However, if the seller has fulfilled the conditions (eg to produce the item for sale by a certain date/time), then the seller can sue the buyer for breaching the contract. Same applies for buyer.
Anyway, all the above is theoretical, in real life, unless you're buying tens of thousands of dollars of product in B&S, why go thru the trouble of suing someone, even in SCT? At best you get restitution, but you will not get consequential or collateral damages. At worst, it's your word against the other party's and unless you have lots of witnesses or very detailed records, it's unlikely you will get restitution at all, so you'll be out of pocket in terms of time and SCT fees and your own litigation costs.