Don't buy tours or treks from people at the bus station, on the streets or at your hotel, instead go directly to the travel agency. These people have a very limited knowledge of the trips they offer, most have never gone into the mountains. Chances are they will charge you more than the travel agencies, they will also most likely lie about what is included in the price and make false promises. These people work illegally and are just after one thing, your money. They have no interest in providing the best trip according to your needs and will transfer you to whichever agency offers them the biggest commission. This means that you have no idea or control over which agency will organise the trip for you. Since the quality of service varies hugely depending on the agency, it really isn't a good idea to let these people make the decision for you.
Instead visit several agencies in town, check the equipment, be very specific about what's included in the price. These 'touts' also work in many hostels. Very few hostels have their own travel agency. Make sure you get a receipt that has the name, address and RUC number (11 numbers) of the agency. Also don't accept lifts to hostels from the bus station. Most of these hostels are located a long way from the town centre (such as Ishinca Hostel), and they hope that you will buy trips from them since it's a long way to town.