Make sure the seller has 100% rate. Offer PAYPAL ONLY and if the seller has a link for Square Trade so you can buy insurance in-case it goes out. But the real test is to ask the seller for the serial number and he/she has to provide pictures of their OWN machine. 9 out of 10 times that still doesn't mean anything till you start some communication with the seller. If the seller is good, they would be available to give you all the info you ask with no hesitation.

There are some good sellers who have limited knowledge of what they buy, but will state that on the item they sell. A good seller will also state if the unit they are selling has some problems and may suggest that the unit be salvaged for parts. Ask the seller if they bought the unit from someone else used or if they purchased it from MAC when the got it. This question is important as you don't want to purchase other people's problems. Some units that are used will have slight scratches on them because of normal use. 9 out of 10 times it's only cosmetic and wont effect the operation of the machine. You may want to ask what are their return policy (if they have one).

Avoid sellers that charges outrageous shipping prices. I've seen some sellers with awesome machines, in great condition charge up to $100.00. For a G4 that's way too much. The most any seller should charge for shipping should be at the MOST 5-7% of their value (that also depends on how fast you want the machine). Some will charge you for handling. One seller I bought from charged for shipping and handling...meaning the seller had UPS packed the unit. I told the seller to provide me with an estimate before I purchase the unit. I did that to be sure that the seller wasn't making money from the shipping as well. You'll get sellers who'll do that and you'll wind up buying a unit that's more than the value of the unit itself. Every little detailed question helps before you purchase.

You'll have to do some homework yourself. It's not just the seller who has to know and be trusted in what they are selling.

You'll have to ask yourself if buying the machine is cost-effective, especially if you want to upgrade. All this depends on what you need the machine for. If your looking be cost-conscious but would like to do some hi-end editing and graphics, you may want to stretch in purchasing a low-end G5.

G4's still have a lot of life in them and can run some of the current programs running today. You best bet is to take a look at The Mirror Drive G4. This unit has the least of what you need to keep up with some of the current programs that are in the market today. What's also neat about G4 computers is that some MAC owners use them as servers when they've upgraded to MAC PRO's.

Good luck with your purchase and stay on top of what you look for!