They are the bane of my existance those hammock hawkers and snake oil salesmen who hang downtown around the main square. So my advice is take a page from Nancy Reagan and JUST SAY NO! Well if you cannot just say no then, say "Yo Tango" which means I have one and keep walking. Walk or Drive over to Hamacas El Aguacate a family workshop with a large selection of very good hammocks of various qualities and colors. The prices are posted and they are fair and correct. There are alot of tricky dealers around in the historic center and recently they talked our guests, Mo and Katinka, into buying a $20 usd hammock for $160 usd with some story about hennequen and vestal virgins or some such. There are hammocks made of hennequen and some made of the plant "lengua de vaca" (sanseviera). If you want one of these go see Louisa Vogel on Sunday's in the Santa Lucia Park (she speaks english, spanish, french) or go to Casa de las Artesanias on calle 63 between 64 and 66. But if you want a cotton or nylon hammock go to Aguacate Calle 58 at 73. you don't need to spend more than 20 to 50 dollars for a hammock unless you know what you are doing. Hammocks come in 4 sizes sencilla (single), doble (double), matrimonial (queen size), matrimonial especial (king size). How to buy a hammock... First of all, never buy a packed hammock without checking it. They are handmade and the craftsmanship can vary in quality. Besides, you want to make sure you are getting what you are paying for. To judge the quality, hold the hammock out by its loops and run your hand over the weave, pushing down slightly. The weave should then close under your hand in a firm fashion. If it's flimsy and your fingers slip between the threads too easily, the work isn't up to standard. The end strings can also be counted to judge the quality; a good size hammock should have at least 200 pairs of end strings. The end hanging-loop should be thick and firm, not thin and be thick and firm, not thin and bendy.
The nicest hammocks I know of come from the prison on the libre road between Valladolid and Chichen Itza! Tixcocob is the town where a lot of hammocks are made but once again you'll have to haggle.