For a discussion about why you would want to use trailers, check this list of reasons.

Also, see for lots of information on trailers generally, plus user reviews of the models which that website sells.

There are also user reviews at mtb REVIEW. In the SEARCH box at the top of the main page, just type the word trailer, then click the Search button.

To choose a model of trailer to buy, check this trailer comparison chart.

I sometimes see used bike trailers advertised in online classifieds, for around US$60-100. These are almost always child-carrying trailers, perhaps used for weekend rides. Even if you don't have kids, you could buy a used trailer just to see if you like using one to haul bulkier items. You can always buy a more appropriate trailer later.