I have put together a oneshot which basically comprises of the basic scenario and a string of encounters with homebrew monsters and culminates in a boss fight. It's kind of a novelty oneshot (you'd probably only play it once, though you might run it multiple times for several groups). How much detail would you expect or want if you were paying a little bit for a one-session adventure? I'd like to put it on DMsGuild and make it pay what you want, but I'd also like to make it so people want to pay a little for it!

Currently I have a description of the town, but no maps or individual buildings described. I have a basic description of the festival, and what the stalls sell, but again not in any great detail, just scene-setting stuff. And I have the description of the event that leads to conflict, and then a string of encounters which leads the party to the boss fight. The document has the monster stat blocks for each fight, and an encounter table which dictates each fight based on the number of PCs.

I don't want to overload it with detail, s it's only going to last one sssion and I wouldn't want to read 40-odd pages just to run one session, plus I want the DM's to be able to make their own twist on it. I am thinking that maps are a good option, as they will help to draw up battlemaps. I'm going to add some art in, because to me a document is empty without any, particularly for the monsters as a picture makes it so much easier to imagine.

What else makes you think "you know what, I will pay a little for this!"? What would put you off?

For a one-shot scaffolding like that is definitely useful. I would second adding art and maps to help give an understanding of spatial relationships and to help run the thing. In terms of paying, probably having one or two well-fleshed out NPCs you can drop in as bad guys or instigators of the plot would be great. I think it comes down more to making it a little more obvious than you might normally how much work has gone into this - for me, high-quality maps of new locations would definitely get the wallet out in appreciation.

I'll have to do some online learning for high quality maps then! I'll also need to make some NPCs to be important, as it's kind of just an event which takes place around them and relies on their innate desire to save the day right now.

Well there'll be a few easy ones you could flesh out when it comes to the events the characters respond to - maybe one of them knows the boss or whatever. Don't change the plot too much, because it sounds like the story is a really interesting one to play anyway. Good luck!

So it's up to 7 pages now, without any gaps for pictures or maps, and I'm calling it roughly done! I'm trialling it on Tuesday so I'll gauge the effectiveness of it on how much I need to improvise!

That is a subjective question.  Mine can range anywhere from a 1 page outline with minimal details, to 20 pages full of details. 

It really depends on what you're trying to do.  

I have a one-shot adventure in old wizard's tower ruin that has exactly 2 small combat encounters and a myriad of puzzles and physical feats the players must answer / traverse to reach the inner library to obtain the long dead wizard's journal.  That is 17th pages long.

I have a dungeon crawl that has a randomly generated map and randomly generated monsters that is 1 page in size.  The page basically only has the premise of why they are in the dungeon.