Nice layout, seems well behaved, and is easy to navigate. The ad categories might benefit from some tweaking. For instance you have several categories related to housing but they are not grouped together. The lack of a travel related category on an international site is rather glaring. Some of the categories are very broad such as electronic and spareparts, while others are very narrow such as new books. I for one would not want to browse a bunch of ads for cameras if I was looking for a computer. Although it ads a whole layer of complexity, you should consider sub categories to better classify the ads. For example housing category could encompass apartments, houses, student apts, perhaps further broken down by rentals versus purchase.
As you are using English as your primary language, you should ensure that your categories use standard english useage: For example Electronics not Electronic. Generally you will find the plural version of a word is best for most categories.
Good luck with the site!