When buying over the internet the chances are you won't have had chance to see the exact model in action. This means you have to look at and understand the specifications.
One of the factors that dictates brightness is the size of the LEDs. The 3 most common types of LEDs found in LED lighting fixtures are 5mm, 10mm and 1W. Budget LED PAR Cans often have the less powerful 5mm LEDs. These will look bright to look at but won't cast a huge amount of light on the surface you aim them at. PAR cans with 10mm LEDs aren't much more expensive and are a more realistic replacement for standard PAR cans. 1W LEDs are the most powerful LEDs that are commonly found in entertainment lighting and can offer a similar output to standard PAR cans but they are significantly more expensive.
The other main factor in the light output is the amount of LEDs. For PAR cans with 5mm LEDs you should look for ones that have a minimum of around 150. Anything less and you're likely to be disappointed. Some manufacturers will use 10mm LEDs but to keep costs down they use fewer. Again you should look for at least 150. 1W LEDs are physically bigger and have very large light output. Around 36 will give significant light output.
If you can't find details about the size and number of LEDs you should stay away. If the information isn't included in the product description it's probably because the seller doesn't want you to be able to compare it to superior models.