In my opinion, and in my experience, there are three reasons for going to the ebay website. Each of these three reasons will probably have quite a large and perhaps even a critical influence on your buying stratergy.
Firstly, one may log onto the ebay website because there is a particular item that they are wanting to find. An example of this might be a coffee machine. In such a case, one will have a budget in mind and in all cases, just as when shopping 'traditionally' in an actual shop, one m,ust be aware of the upper limits of their budget.
Secondly, one may be looking for a general item; maybe an item of clothing, but the exact item is not specific. If the above example is directed, this example is more leisurely and recreational. Naturally, be aware of your budget, and try not to strtch it too far. A good hint would be to combine items from one vendor so as to minimise shipping costs.
The third reason one may log onto ebay is just to have a look. This is basically window shopping or simply brousing. One will sometimes see something, but not always.
In conclusion, no matter what the reason, always stay within your budget, and don't forget to consider shipping costs. As a final note, be aware of international sites from which a specific item may be easier to find.