Changes-as above, that ONLY card payment is accepted for tickets sold on the vehicle are underway soon, as also predicted.
ARRIVA will move over to No cash from either 1/12 or 10/12 coming (thats the europe-wide change of timetable sunday). Also for the small volunteer driven ''buurtbus''=community bus (to small villages where big buses are just too big-Province of Noord Brabant has nearly 90 of them).
Connexxion ditto, except for an odd exception. Syntus will be renamed Keolis and ditto to.
Lagging a little behind is QBuzz for Groningen+Drenthe, no info from EBS (between AMSterdam and Hoorn-a very heavy commuter network).
And again: the main method of ''paying'' is by that OV-chipcard. I've spent quite a few long days lately on country buses and there are whole days where the driver does not get one single cash payment. On short peak-only duties they often do not even bother to take the tickets+changecash out. Understandably you from abroad seem to worry, but all in all its even much more customer friendly as what TfL buses currently can offer.
A peculiar case will be on the sectors of international busroutes in Germany (mostly ARRIVA too): I really dont know if this is even allowed in DE and yes, they there do sell loads of cash tickets.
Here in NL there is NO legal need as such to allow any form of payment-as long as your clearly post what is accepted-most shops wont accept notes over 100€. (nor do banks give them out, except on special advance request).
A similar discussion about this already in force on GVB-buses was on this forum recently- what I recall most seemed to think it would be contactless only (like in LON-I assume your only example till now in the UK) and thats just not true. EVEN US cred-cards are accepted (but not AmEx).
Another thing in travellers-societies lately is that many buscompanies (notably again ARRIVA) now want to raise on board ticket prices as this payment is more costly to them as cash.
(in most cases the brunt for those tickets is laid on drivers: they get 1 stock of each of them when they start job and just have to take care of replenishing them by themselves-and then paying in the due amount-when they are in depots). In most cases the nr of different tix as such is limited to 2 or 3. Strangely new-style ticketmachines (which can easily be combined with check in for chipcards) have hardly gained favour here-just 1 or 2 concessions use these.
On TRAINs-after all there is this forum for-sale of tickets is not possible since many years now. The ONLY way to do that for conductors is to write down personal details from ID, the fare and the legally prescribed former 35, now 60€ punishment fee-to be paid afterwards by bank and in time. In genuine dire straits, to be checked by NS afters, the fee can be paid back-on discretion of NS only.
Recently even those conductor held stamping things are now abolished and replaced by electronic control checks on chipcards. This makes claiming back more as justified on long trips by OV-chip more diffult (you just forget to check out after a long trip and afters claim to have exited at a nearby station without barriers-then you get the balance back).