I've seen lots of eBay auctions for guitar amps with wattages listed that just don't sound right...  A transistorized practice amp with a single 8" speaker rated at 38 watts?  There's an important lesson to be learned here.  There are often two wattages listed on the back of a guitar amplifier.  One is the amount of power that the amplifier draws from the wall socket, and the other is the output power.  The SMALLER of the two is the output power, and the one you're probably interested in as a musician. 

As an example, a practice amp that I own is labeled on the rear: "20 WATTS, 9V RMS, 4 OHMS" and "120 VAC, 60 Hz, 50 WATTS."  The first list of specifications is what the amp outputs to the speakers.  The second is what the amp draws from the wall.  Listing this amp as 50 watts in an auction, while not a complete lie, is highly deceptive, as most people would believe that the amp OUTPUTS 50 watts, when in fact it outputs 20.

If you're not certain, go to the manufacturer's website, that should be able to give you the real specifications of the amp.