If you need to buy something at a good price, or sell something quickly, there's no place like the classifieds. Most print newspapers have a classified ads section, their rates vary from $10 to $20, depending on the newspaper. You should also check your supermarkets for free newspapers, that often publish local ads free of cost.
Shopping centers have bulletin boards where you can buy and sell stuff. Just be sure to get it stamped by the administrator before you pin it, or you ad will be taken down in less than 24 hours.
If you want to go the online route, there's several sites that offer classifieds, usually for free:
Mercadolibre is run by Ebay, Buscomi is run by Grupo Nacion (a local newspaper), and Encuentra24 is an independent project, run by a panama-based company.
Getting your packages to the buyer is normally done in person. Distances in CR are short enough for most people to come and pick stuff up. But you can also use EMS courier, or registered mail, both of which are offered by Correos de Costa Rica at any post office nationwide.
You shouldn't use Ebay to sell your stuff in Costa Rica (use their local version, Mercadolibre). You'll have a hard time getting your money from Paypal (the only local bank that supports Paypal is Banco Nacional), and if your buyer is outside of Costa Rica, shipping costs will be astronomical, even through the national postal service. You should only use Ebay if you're selling a highly specialized item (i.e. limited edition art or collectibles), that has a high value and will sell well enough to make up for the international shipping costs.
Online sites will also let you buy and sell cars in Costa Rica, although for cars, the most well known site is CRAutos