We don't know enough about the particular circumstances to say whether or not the dealer was behaving reasonably. Most dealers have used gear as trade-ins on a purchase of new gear. Taking in trade-ins is a BIG favor to customer and the dealing with the trade-in is a big PITA and often of marginal profitability. One cannot expect the dealer to be quite as helpful with a used sale as they would be with a new sale. If the trade-in model is current and of high value, it might be an easy sale, in which case, there is little incentive to unbox and set it up for any one of many prospective customers. The amount of effort to make the sale is a business judgment based on the dealer's business practices and the particular circumstance. The dealer would have to be concerned with tire kickers coming by to get a free "audition" of something not normally available in the area.
Ozzy62 makes a good point. If both the buyer and seller have some skin in the game, there is less concern by the seller that he is just being used. You should offer to pay a refundable fee to audition the amp.