Complete and utter rubbish...
Not untraceable at all - in the UK at least, any substantial cash purchase is likely to raise warning flags with the bank when it's deposited as the bank is an FCA regulated authority.
If someone deposits GBP 26K, you can bet that the bank will want to know where it came from, and why - it's a criminal offence for the bank not to do due diligence (KYC) under the Anti Money Laundering regulations as they are FCA regulated - other bodies who may deal in cash are also covered, regulated or not see https://www.gov.uk/guidance/money-laundering-regulations-introduction
Anyway, what a prattish thing to do - giving the rude salesman the commission on a sale he patently didn’t deserve....
On an almost identical note, many years ago I had been working as a contractor abroad and had a nice pile in the bank. I fancied a new BMW M3 (this was before I was married...), so went to the local BMW dealer, L&*, and had a browse - I was wearing jeans & trainers. Long hair. Bit grungy - dunlop green flash ISTR (them was the days).
A salesman ‘executive’, let's call him Dominic, came over and asked what I was doing there as (and I paraphrase here) I didn't seem to be the sort of person who would own a BMW. I was incensed.
I left. At an NFU BBQ a few weeks later, we arrived in my new M3 bought from a dealer in Scotland who had put the car on the overnight train from Edinburgh to London, driven down to our house in the SE and did me a deal. Amazing service.
So, at the BBQ, I met with the owner of the BMW dealership, let's call him Tim. Tim asked about the M3 as he had one (mine was silver, his was red - he also owned two M1s) and where I'd got it from – he knew it wasn’t from L&*…
I told him the story - Dominic "left" a week later.