In Canada it is illegal to influence upward or discourage price reductions to the customer either through agreements or threats or any other influence. However, everything else is up for negotiation and contractual agreement.
So, it is legal to have an MSRP, only allow advertising MSRP, require the carrying of a full line, etc. Not legal to restrict the price charged to the end consumer. But, your contractual relationship can help you find cooperative retailers.
Retailers generally are not interested in losing money. Good retailers understand that there are customers that are too expensive to service and have policies and procedures in place to try to avoid and/or fire those customers. To that end, few retailers are interested in competing solely on price. This is why
Sweetwater has those people that phone you - their job is customer retention. Likewise, they advertise their inspection of the instrument before sending it out to you as value added that will influence you to buy from them regardless of price. How about free shipping; ever noticed that free shipping only kicks in at a price slightly more than the cheap item you were going to buy? You either have to pay a profit margin on shipping or buy more to make sufficient profitable purchases to qualify. It is a means of discriminating against the 'deadbeats' that cherry-pick prices (i.e. firing those unprofitable customers).