In Ancient Rome at least, there were professional sign-writers whose job presumably consisted of writing signs to advertise for political candidates, games and maybe also businesses.

The surviving examples that we can directly link to such a scriptor are all notices advertising election candidates and games organised by wealthy citizens, but don't I think that it precludes them from also working for businessmen, who often advertised their. products as well.

Here's an example from Pompeji:

CIL IV, 3884

D(ecimi) Lucreti / Satri Valentis Flaminis Neronis Caesaris Aug(usti) Filì(i) / perpetui gladiatorum paria XX et D(ecimi) Lucretio Valentis fili(i) / glad(iatorum) paria X pug(nabunt) Pompeis VI V IV III pridie Idus Apr(ilis) venatio legitima et vela erunt // scr(ipsit) Celer // scr(ipsit) Aemilius Celer singulus ad luna(m)

Translated: "20 pairs of Gladiators of Decimus Lucretius Satrius Valens, perpetual priest of Nero Caesar, son of Augustus, and 10 pairs of Gladiators of Decimus, son of Lucretius Valens, will fight at Pompeji on the sixth, fifth, fourth, third and second day (counted backwards from) the Ides of April (8th through 12th April 79). There will be a legitimate hunt and awnings! Celer wrote this. Aemilius Celer wrote this alone by (the light of) the moon."

Aemilius Celer is mentioned as scriptor on two other electoral notices (CIL IV, 3775, 3820). Like any good advertisement agency, he is advertising for himself as well through his work.

Another such advertisement (CIL IV, 1190) ends thus:

Scr(ipsit) / Secundus / dealbante Vic[tor]e / a{d}stante / Vesbino / [red]em[p]tore [

Translated: "Secundus wrote this after (the wall was) whitewashed by Victor, under the presence of Vesbinus, contractor"

Combined with the above Aemilius Celer mentioning working alone, this probably means that they were usually working in teams. Working 'under the moonlight' seems pretty logical, since such posters were usually located on the main thoroughfares, to get the most attention, so they could work relatively unhindered and without hindering traffic or business themselves. The dangers of this line of work are illustrated by the (humorous?) addition to an electoral notice CIL IV, 7621 - sorry for the potato quality:

C(aium) Iulium Polybium / aed(ilem) v(iis) a(edibus) s(acris) p(ublicis) p(rocurandis) // lanternari tene / scalam

Tranlated: "Caius Iulius Polybius for Aedil for the care of the public roads and sacred buildings! Lantern boy, hold the ladder!"

The letters in most of these advertisements are also done pretty carefully and comparably well (the above illustration is not of a very good quality), a far cry from the sometimes barely legible scribble that can be found on the walls of Pompeji as well, by non-professionals (as another excuse, the, often NSFW, subject matter of many of these graffitis suggests that they were written in an inebriated state - some things just never change).

What becomes clear is the name of the ones sponsoring the games has a very prominent place. In fact, the 'D(ecimi) Lucreti' is painted in letters at least ten times as large as the rest of the text. Thus they served not only as notices to the public when and where the games would take place, but also as propaganda on behalf of the sponsors. Euergetism, the sponsoring of public buildings, games and festivals was an important part of elite competition and distinction in those times.

An example for a political campaign poster would be this (couldn't find a good picture):

[CIL IV, 120]

M(arcum) Cerrinium Vatiam / aed(ilem) o(ro) v(os) fac(iatis) / scr(ipsit) Infantio / Fabius Eupor rog(at)

Translated: "I ask of you to vote Marcus Cerrinius Vatia for Aedil (a municipal office). Infantius wrote this. Fabius Eupor asks (that you do this)."

As mentioned in the second example, the wall would usually be whitewashed first (thus also covering previous dipinti), and then painted with the letters, usually using a bright, highly visible red colour. Consequentially, the ephemeral character of these inscriptions also means that we have not very many of them surviving. That the scriptors often used surfaces for their advertisement that weren't intended for that purpose by their owners is attested by various inscriptions, like this one from Narnia (CIL XI, 4126):

Ita candidatus quod petit fiat tuus / et ita perennes scriptor opus hoc praeteri / hoc si impetro a te felix vivas bene vale

Translated: "So may your candidate become whatever he seeks, and so, writer, pass by this eternal monument! If I gain this from you, may you live happily, farewell!"

Or, less prosaic and more direct (CIL X, 3785):

hanc aram / ne quis dealbet

Translated: "May no one whitewash this altar!"

Tombs and the like where probably a prominent target for such advertisements, since they were erected on the side of the major roads to and from the city (burial inside the city limits was usually prohibited).

There were also, of course, professional stonemasons, and permanent advertisements done in stone, but they also did a ton of other different inscriptions whose purpose was only advertisement in the loosest sense.