Ok, many people know that eBay is home to lots of musical equipment.  This is good and bad.  There are tons of great deals out there to be had if you are patient and you research what you are looking at before you commit.  Know your price limits and what you are going to be using the amp for.  There's no reason to go further into debt just because you felt that you just had to have that vintage Marshall.  Chances are that something else might have actually fit the bill.  Look into what makes up the amps that you like and research similar amps.  You can find out a ton of information by just looking at what kind of power tubes an amp uses and the wattage that it puts out.  A handy place for looking into things like this is harmony central.com.  There's a link on the first pages that says user reviews.  You can go there and look up just about any kind of amp, pedal, or guitar that's ever been made.  Read all the reviews of a certain product.  It will become obvious which people know what they are talking about and which ones are beginners who don't knwo what they are looking for yet.

Once you have an idea what kind of amp you want, you can start looking for one on eBay.  I suggest setting up a few favorite searches.  With this, you will get e-mails everyday if something with your keyword has been listed.  It's simple and effective. 

Another thing to do is go to eBay and search within your cash parameters.  If you go to eBay, click musical instruments from the first page and then click amplifiers from the second page.  You will then have a list of options on the left side of the page that has an option near the bottom so that you can search between certain dollar amounts.  Enter the lowest amount that you will pay in one box and the highest amount in the second box.  Hit enter or search.  The results will come up either as newest listed or ending first.  I find the best results can be had from either option.  Sometimes you'll find something that's about to end that slipped through the cracks, sometimes you'll see something that was just listed for a great price.

Once you find an auction that appears to have what you might want, click it.  Look at the seller's feedback.  If they have a ton of feedback and the percent positive is close to 100%, chances are that they are legit.  Scroll down through the auction.  Are there enough pictures so that you can tell what you are buying and the condition of it?  Does the pictured condition match the stated description?  If so, chances are that this might be what you want, but wait...  Don't click anything yet.  Look at the shipping.  Do they even offer shipping?  Are they trying to charge too much for shipping?  This is where most transactions come to a screeching halt.  Some people seem to think that they can make up their loss on a lower price for their item through higher shipping charges.  When in doubt, e-mail the seller if the amount seems high. 

Once you have determined that the shipping is available and hot highway robbery, it's time to bid away!

Good luck!