You could also have a look on line at one of the many stores these will give you a further idea of the new price.

If the pedals are in tip top condition expect to pay between 50-75% of the new price.

If this is a private sale you may also want to gently remind the person selling it that if they were to sell these things on consignment the store would take about 25% of the sale price !

The Boss flangers (had two both great !) and Blues Drivers are nice but avoid the Boss GP-20 and OD-20, they suck big time !