If you are paying cash of more than (ironically) Euro15,000 (circa �10,000) then there are money laundering regulations to be considered.
If there are 'one off' transactions like nox's bike, there are no real issues - though the cashier may ask the question: "where did this money come from".
Companies such as car dealers have a far more difficult time though - as you can imagine lots of drug dealers (and others who want to money launder) going in to pay cash.
It would be fairly sensible for a garage to refuse to take large amounts in cash - at the company I used to work for (a finance company) we refused to take someone's cash when they wanted to buy a repossessed limo - they had to transfer the money into our account.
Under money laundering regulations, there is not only an onus on the person taking the cash (ie the salesperson) to report any thing he feels is suspicious, but the salesperson can in fact get fined / go to prison if there are subsequent issues (rather comparative to serving an underage person in a pub - bar persons fault - not just the pubs)