Could you explain the reason for number 3? (Especially since you don't seem to mind it when a game has almost no graphics; computer versions of Wizardry come to mind.)

Also, could you explain number 4?

(1 and 2 I understand, though being a beta tester can be fun if you are the sort of person who enjoys hunting for bugs.)

My major rules (have some more, but these are the main ones):

1) No DRM, if it requires any DRM to play, it's a "Do not Want, even if it was free"
2) No GiD, EA, etc... No need to pay for the "honor" of being a beta tester
3) No Day 1 Releases, I usually wait half a year to a year when most bugs are fixed and the price finally went down.
4) No Episodic titles until at least the whole season is out.
5) Only buy at a cheap price / high discount... don't have lots of money to throw around willy-nilly.
6) Check some Let's Plays / Youtube Reviews to see how the game performs and if it tickles my interest. Much harder to fake a "positive impression" when the game in the background still looks like ass and runs like shit.
7) Somewhat synonymous with 6), No blind buys or getting onto hype-trains. Most games usually don't live up to it anyways.
8) Only buy games I'll eventually play, unless contained in a bundle where buying all games is actually cheaper than only those I want to play. Happens very rarely though.

Here are a few others of mine, though they tend to be more guidelines than hard rules:

+: The main character (or a majority of the selectable ones, including the default one) is female
+: The main character's gender is non-binary (Undertale is the only example of this I see)
+: I like the game's music (note that the use of electric guitar tends to disqualify games from meeting this criterion)
+: The game respectfully portrays LGBT characters (but see below)

-: The game has you create/name a character, but that character must be male (Eschalon Book 1 is one offender)
-: The game is story heavy and the main character, who has an important role in the story, is male (of note, I don't mind games like the Mario series where there isn't much story, though I think you can guess who my favorite playable character in Super Mario Bros. 2 (USA) is).
-: The game contains significant amounts of sexism, especially if it can't be worked around with mods or cheats
-: The game contains homophobia/transphobia (transphobia has a good chance of being a dealbreaker here)
-: The game contains offensive portrayals of LGBT characters (another likely dealbreaker; if you don't know how to portray such a character, it is better to leave the character out)
-: The game uses Baldur's Gate style real-time-with-pause combat
-: The game plays in real-time and forces you to rely on AI control for your companions (oddly, I don't mind this in non-tactical turn based RPGs such as Dragon Quest 4 and Wizardry 4)
-: The game is cutscene heavy, especially if there are a lot of cutscenes early on
-: The game has music that I detest (hard rock or anything similar)

I only have 1 rule when buying games: never letting my wife know how many I bought or how much it all cost!

This is important to me too, and I find it's becoming more and more of a hard ruleset for me -- though for me, it applies to modern indie games much more than classic games, and it's usually more about whether there's an option to customize a female character than whether there's a default female lead.

If you're interested, I started on a couple of mixes awhile back that mainly deal with how games treat female characters and whether you can play as a woman, though these lists aren't yet as extensive as I'd like:
https://www.gog.com/mix/gender_equality
https://www.gog.com/mix/gender_inequality

Why, are you are a misandrist?

- no gog = no sale - I like collecting games, and I fully intend to enjoy my collection when I'm old and senile. This absolutely excludes DRM, there is about zero chance that any of current drm will work on windows 2050. While this doesn't technically exclude other distributors, with GOG I need no futher research to make sure. It's only an afterthought elsewhere, a logo maybe placed on the game page. And maybe placed in error, some devs have weird notion of what is or isn't DRM. Or it could be "mandatory upgraded" to use DRM later. GOG selection could be better, but for now it's good enough to not look elsewhere
- only games I can see myself playing eventually - that means no sport simulators, no car races, and be very very careful with anything that claims to be rogue like / lite
- no buyers remorse refunds - if I make an wrong judgement, live with it and learn

I don't exclude anything based on age or sale method, those are all temporary characteristics. I tried them all (AAA preorder, in-dev, DLC) and have yet to be truly disappointed.

I don't have a lot of hard and fast rules. I'm fairly flexible as far as price goes, although I won't buy a game for full price if it's a genre I'm only marginally interested in.

- I don't buy games that are going to have a lot of DLC until it's all out and I can get it in one package deal. The exception is CDProjekt's games, as they've proven to (so far) not be trying to nickle and dime the customer for a crapton of... crap.
- I tend not to buy day one just because it seems new games almost always go through a lengthy and massive patch cycle and I hate having to re-learn the mechanics/character builds/etc after major patches.

- any game I know I'll want to replay has to be DRM-free so I can easily back it up. Others, that I'm not sure about and just giving a shot can be Steamed or Origined, but those are purchased only on deep discount. Having said this though...

...more and more, I'm tending to wait before buying anywhere else but here, as GOG seems to be getting a lot of games I want/have been watching so I'm just starting to wait for DRM-free and say 'screw it' if it doesn't come here. There are so many games in my backlog it's not like I really need to buy more.

- I put a budget before buying anything (usually between 30-50$ max)
- Take advantage of discounts

System Shock was the only time where I bought a game on day 1.

DRM free, that's it.

I'll buy the game if it interests me, that's about it.

I try to keep it simple.

1) No DRM-infected games or game rental stores
2) Easy to backup like GOG installers
3) Watch gameplay videos or try before buy
4) No multiplayer-only games
5) Games with limited story or old games gets low priority
6) Buy when price is at acceptable level

Does it look fun. Every great game I ever bought. I bought off of looks and looks alone. I do not care about any of that anti-DRM nonsense and other weird stuff others before me have posted. Life is too short to restrict yourself of pleasure because of something as trivial as DRM. I remember looking at the back of a PC game box. The great game covers of Fallout, Diablo, starcraft and Command and Conquer especially Tiberian Sun. I use the same rules for buying food. If it looks good I'll eat it. I don't care what it is or who sells it.

Never pay more than 20 bucks for a computer game.