It is not enough to just think in terms of 5V/USB and mA rating. Some chargers out there are really terrible junk that shouldn't even be used for anything. Some are overrated. Some have quite high ripple. If it seems like it's dirt cheap, wonder why.
Any USB power source that accurately claims 500mA can charge anything that is supposed to be USB charge capable, 500mA is the upper limit for USB compliant devices, but it doesn't necessarily mean a device won't get by ok with less than that, things with small Lithium cell battery packs taking multiple hours to charge can't possibly do anything with so much as 500mA current. For example if you have a 600mAH single cell (~ 3.7V) battery pack and it takes 3 hours to charge, it can't be using much more than 200mA (a few dozen mA more to account for inefficiencies but a few dozen mA less potentially if it has a switching step-down circuit (5V->4.2V peak) for charging instead of linear).