The main difference for me is that in the past when going to a supplier or reseller I would presume they could listen to what was requested, make a note of it and quote me correctly. The first fault lies in the fact that the quote was wrong. I've not said the OP was faultless but it doesn't hide the fact that if the quote is wrong then that will bugger everything else up.
Not all orders would be prepared by me either ... I would request a quote, it would come in and I would pass it over to someone to do the paperwork. In the past I have experienced resellers who will put a part number down but not a complete description. At this point you rely on the knowledge and expertise of your reseller ... and some trade on the fact that they are so knowledgeable and experienced. When they make a cock-up on the quote, leaving me or my school out of pocket I have two choices, talk with them to come up with an amicable solution or express my dissatisfaction and take my business elsewhere. I would probably attempt the former before doing the later. When schools have got such a range of possible suppliers, all giving really good prices ... the fact that I might save thruppence by using one supplier does not mean it is best value ... not when there is the cock-up factor to consider. That is why I keep repeating to people that the cost in money is not always the total cost. In the OP's case it is also the disruption caused, in some cases it is the haggling that some companies insist on before they give you their best price, or it might be lowering your ethics by passing on prices from another supplier so that they can be beaten by the company you prefer or have friends in. I know not everyone would agree with all those points (especially the last one ... some folk don't understand why I have an issue with this) but if a company cannot quote me correctly then I move on ... saying that it is just the OP's fault is not fair to the OP.