It is common to bargain for most purchases in the open air market. Though you may be able to bargain in other places, be aware that chain-owned shops have fixed prices you are no more likely to bargain in a Guatemalan Radio Shack than an American one.
These are some characteristically Guatemalan things you might consider buying here:
The local currency is the quetzal plural: quetzales, the national bird, with ancient and mythic connotations even today. One US dollar is equivalent to 7.61 quetzales. US dollars are widely accepted and be exchanged in most small towns. ATMs can be found in the major towns but do not expect to find them in every tourist spot. It is fairly easy to find your self in a town without an ATM or a place to change money.
Do not expect to be able to easily exchange travelers checks to Guatemala. You might find a few places willing to accept checks issued by American Express but other types are universally turned down. Amazingly, even major banks in Guatemala City do not accept VISA travelers checks.
The national currency is the quetzal. The rate of change is approximately 7.61 Quetzales per US dollar and 10.88 per euro May 2011. It is common to use dollars in tourist areas. You will most likely have difficulties in changing other currencies than US dollars, but euros are becoming increasingly common.