Without knowing your location, and which system you are trying to monitor, it would be difficult to give you any useful recommendations. It could be the scanner, or possibly the system you're trying to hear is simulcast.
As far as charging goes, to scan and charge at the same time, the HP-2 needs a 'smart' power adapter, capable of providing 1000mAh or more, and it should be one that indicates that capability to devices plugged into it. (The USB ports on many, especially older, PCs are only capable of providing 500mAh of power, which is insufficient to both run and charge at the same time.) Make sure whatever you are using meets that power requirement. If it does, but the scanner still will not charge (especially if it won't charge when turned off (not scanning), then the unit may be defective. Are you using the original batteries that came with it, or known good ones (new or otherwise). If the original batteries are being used, they could be past end of life, and the reason that they will not charge or operate the scanner. You can verify whether the scanner would run on batteries at all if you inserted a set of new alkalines. Of course, you should not charge them, but that could verify whether the problem is a disconnect somewhere between the battery holder and the scanner internally.
Presuming that whatever previously owned scanner, HP-2 or otherwise, is working correctly, there would be no difference in operation between a used or new scanner, other than perhaps a few scratches or scuffs on the used one.