I don't know if it's a red flag but if they are contractors, they may get contractor pricing at those places allowing them to buy a pump much cheaper and make more money off of you.
Speaking as a contractor......Lemme see, you call a Pond guy, he comes to your house and does an assessment, he recommends a pump that he'll back, he gets it, pre-pays it, delivers it, removes the old one, installs the new one, registers the warranty and then comes back and hauls it out if something goes wrong and you begrudge him making, what.....maybe 15-30% from the product sale?
You don't think that the retail outlet you're buying it from makes any $$?
For instance, an AquaSurge 4000 2nd Generation pump retails for Can.$383.99, wholesale (which is what your pond guy gets it for), is $359.99. That's a whole $24!
A 1/4hp large pond (1 acre) aerator retails for Can.$700, wholesale is Can.$560.
If you want to get ripped off, go to a car dealership or get a real estate agent.................$24 won't buy a box of beer up here. Peanuts..........