When it comes to buying used equipment, if you are an experienced screen printer and have purchased new equipment over the years, you will definitely have an advantage over someone who has never printed before.
If you are new to the business and have no idea how a piece of equipment is supposed to work, it is highly recommended that you take along a friend who has some screen printing experience to see if the equipment works as it should.
The best kind of used equipment to buy is from someone who started up a screen printing shop and failed in a short amount of time. Usually the equipment was not used much and if the "ex-printer" has lots of bills to pay, you may be able to bargain into a really good deal.
If you are leary of dealing with private sellers, a good place to look for decent quality, used screen printing equipment is from your local screen printing supply vendor. Often, they will take in equipment on trade or consignment and they do not want to damage their reputation by selling you equipment that you will not be happy with. Moreover, they can demonstrate how the equipment works and provide you with supplies and other useful advice as well.
When it comes to prices for used equipment, do some homework. The internet is a great place to do some research when it comes to finding the value of used equipment. Check out Ebay and Craigslist. Do a search for "screen printing" or "silk screen equipment." You should be able to get a very good idea of what the equipment you want to buy is really worth.
Bottom line–before you buy anything, ask a lot of questions. Insist on seeing the equipment in operation and ask the the seller if they will stand "behind the purchase" if the equipment should fail in a short amount of time.