WTB - Want To Buy: Thread that you start where you want to buy something and are looking for offers from people who have whatever it is you are looking for.
FS - For Sale: Thread where you want to sell something.
IC - Interest Check: Thread where basically you're checking how many people would be interested in a particular deal.
GO - Group Order: Thread where a group of people order the multiple quantities of the same thing and thus reduce price.
'In case of a Want to Buy thread, you can mention desired price or write Open to Offers': This essentially means that when you open a WTB thread, you mention the price (range) you are willing to pay, or write "Open To Offers"; meaning if you think the product is good for whatever the seller is offering, you'll take it.
To you original question, you can try EBay or the Member's Market here itself.
