When I visited Gibraltar in 2010 both GBP and EUR were accepted on buses and the fare was £1.80 or €2.40 with change given. €2.40 was about £2.10 at the time I believe. Some of the drivers got flustered when I paid in GBP as they expected tourists to have euros. I can't remember what exchange rate was used on my subsequent trips.
A similar thing happened in Feldkirch (Austria) when I boarded a Liechtenstein bus and proffered CHF, the driver said I should have told him I was paying in francs first as he was expecting euros and had to reselect the ticket on his machine or something. I can't remember what exchange rates were used.
I am surprised that the drivers in Gibraltar would expect tourists to pay in euros. They have an airport so lots of people visit Gibraltar directly from the UK or other non eurozone countries. Last time i was in Gibraltar i paid solely in GBP on buses. Most other people i observed were paying in GBP too but it was mostly locals on the buses i used. I recall i only noticed two occassions of people paying in EUR in Gibraltar who were both clearly tourists.
Gibraltar still accepts GBP and EUR on their buses but the exchange rates for paying in EUR are awful. Paying in EUR results in a much higher fare. For a day ticket you are paying almost £2.00 extra for a EUR purchase. If you want a 40 trip ticket then you pay almost £12.00 extra. They are the worst exchange rates i have seen on any bus route that accepts multiple currencies.
The current bus fares in Gibraltar are as follows (with the current exchange rates in brackets) and they have been the same price for years:
Calypso Transport:
Routes: 5/10/X5
• £1.60 (€1.84) / €2.40 (£2.09) - Adult Single
• £2.40 (€2.75) / €3.60 (£3.14) - Adult Return
• £6.00 (€6.89) / €9.00 (£7.84) - Adult Day
• £10.00 (€11.48) / €15.00 (£13.07) - Adult 10 Trip
• £20.00 (€22.96) / €30.00 (£26.14) - Adult 20 Trip
• £30.00 (€34.43) / €45.00 (£39.21) - Adult 30 Trip
• £37.00 (€42.47) / €55.50 (£48.36) - Adult 40 Trip
• £1.30 (€1.49) / €2.00 (£1.74) - Child Single
• £2.00 (€2.30) / €3.00 (£2.61) - Child Return
• £4.00 (€4.59) / €6.00 (£5.23) - Child Day
• £1.20 (€1.38) / €1.80 (£1.57) - Pensioner Single
• £1.60 (€1.84) / €2.40 (£2.09) - Pensioner Return
Gibraltar Bus Company:
Routes: 1/2/3/7/8/9
• £1.80 (€2.07) / €2.40 (£2.09) - Adult Single
• £2.50 (€2.87) / €3.30 (£2.88) - Adult Return
• £6.00 (€6.89) / €9.00 (£7.84) - Adult Day
• £1.50 (€1.72) / €2.00 (£1.74) - Child Single
• £2.00 (€2.29) / €2.65 (£2.31) - Child Return
• £4.00 (€4.59) / €6.00 (£5.23) - Child Day
• £1.00 (€1.15) / €1.30 (£1.13) - Pensioner Single
• £1.50 (€1.72) / €2.00 (£1.74) - Pensioner Return
Gibraltar Bus Company:
Routes: N1/N8E/N8S
• £2.00 (€2.29) - Adult Single (Euros Not Accepted)
I suppose they must be making a good amount of money on EUR fares from people visiting from Spain who have not bothered with exchanging currency. A lot of shops in Gibraltar seemed to accept EUR too.
One oddity is that the night buses only accept GBP payments. They are the only routes that EUR can not be used on. I suppose they expect that mostly just locals will be using night buses.
It is also always surprising that there are two competing bus companies in Gibraltar running local bus routes despite the tiny size of the place. The state owned Gibraltar Bus Company (1/2/3/7/8/9/N1/N8E/N8S) and also Calypso Transport (5/10/X5) who are privately owned.
I wonder if the Spanish local buses in La Linea De La Concepcion accept GBP for payment. I never thought to ask last time i used those. They must get a lot of people from Gibraltar using them. Their fares are a lot cheaper than Gibraltar at just EUR1.00 (£0.87) Adult Single with cash or EUR0.85 (£0.74) Adult Single with a smartcard or EUR0.75 (£0.65) Child Single or Pensioner Single with cash or a smartcard.
Your mention of Liechtenstein reminds me that Liemobil (the Liechtenstein state owned bus operator) run dozens of cross border routes. They have six routes that cross in to Austria (11/13/14/414/470). They have six routes that cross in to Switzerland (11/12/12E/13/24/37). They even have two routes (11/13) which cross from Austria to Liechtenstein to Switzerland and as a result operate in three countries.
San Marino appears to have just one cross border route (160) which is operated by the Italian based Start Romagna which serves the San Marino village of Gualdicciolo. It runs every thirty minutes from Rimini (Italy) to Novafeltria (Italy) but only two journeys a day (one journey a day in each direction) serves San Marino. The other journeys stay entirely in Italy. I think the two journeys that divert via San Marino are mainly to cater for school students but it is a public route. That seems to be the only cross border local route. San Marino has a fairly extensive bus network of nine local routes (1/2/3D/3S/4/5/6/7/47) operated by the state owned AASS and many of these terminate next to the border post but nothing crosses the border.
There is also the Rimini to San Marino coach route operated jointly by Bonelli Bus (Italy based) and Benedettini Bus (San Marino based) with each operator running half the journeys.
I wonder whether the Vatican City has ever had any cross border bus routes to Italy at any point in the past even if it was just for special events like their train line.