Anyone heard of these painters?

Can anyone help / offer advice, please? I just sold a fairly rare and quite valuable vintage knitting machine and accessories to a business as a private sale to a business. It's run a couple who specialise in repairing and reselling secondhand knitting machine parts. I tested the machine and accessory prior to selling and both were working. The business has just texted me with a fairly aggressive message to say 'it can't be working as part of it is missing' and that the pattern reading accessory does not work. I know that both parts were working when I handed them over, so it is just their word against mine. (I haven't replied to ask what part is missing yet) When they arrived to pick up, the owner said he needed to do 'due diligence' ie to open the case of the machine and check it was all there. He did all that and did not mention anything was missing at that point. They paid by bank transfer on pickup. Unfortunately for me, I got them to pay into my business ac (even though I no longer run a business and am retired, I still have the account) and now they say because they paid to a business ac they can dispute the payment. I did get them to sign a delivery note with a list of items that they were buying  but I did not specify 'bought as seen' on the note. How does it work when it's one person's word against another? I originally found the business online and emailed them with the details and photos. They had good reviews for selling items (but no reviews re: buying items) The owner always replied by phone rather than email so although I have notes of our conversations I do not have anything in writing from him. I'm feeling angry at having trusted them and also upset - textile design was my career before I retired and I wish I'd just given the machine to a charity now1 Any helpful advice much appreciated!  

There are several threads on here about door to door sellers with similar false stories, but I don't know what the thread headings were or how to find them. Someone else may have a better memory! No idea who to contact, possibly the police non urgent number, can't remember what it is, sorry.

My objection is that it is an Americanism. Spoken id prefer Mon to Thurs. In writing Mon - Thurs.  

Couple of likely lads knocking doors earlier this evening, claiming to be from ‘rehabilitation project’ and trying to sell cleaning kit to raise money to get into a hostel. Not really putting much effort into trying to hide that they casing local houses around the library area.  So be it, be aware. But my question is what to do in such circumstances. Should I contact police - who? how? - or the council or something? Would appreciate advice.