The courts are flooded with lawsuits that should've been avoided. Personal injury is only one of the potential nightmares possible in this situation. It happens, a lot. My wife works in insurance litigation, and the crazy things I hear from her are never ending. The owner/manager/whatever has no idea if either party will damage their property, commit fraud, or react hostilely if not satisfied with the personal transaction. If the amp gets damaged while it's present in the store, the owner may be liable. I seriously doubt that in a scenario like this that the music store would have their lawyers provide a disclaimer for OP and his party to sign.
If the owner or manager agreed to this, they'd be setting a precedent for more of this behavior to follow. OP may expect to conduct his business again on their premises, and other customers may witness this and expect to be granted similar rights.
While it's reasonable to think that the transaction could likely go through without incident, it's too risky to take that risk. I would consider someone nuts to allow something like this to take place.
However, I think that the Manager could have been more polite. Clients/customers often have no idea why businesses have various policies, and it falls onto the staff to enforce boundaries in a positive way.