The following is an incomplete list of suggestions for generic considerations that you might like to make when you go out to buy a thing. I have tried to put the list in order; being generic - certain things will be more or less important in different orders.

0. Do I need the thing? Am I just wanting it on a whim (you are allowed to do that, but at least try to not do that for many expensive things that don’t have resale value)? If a month had gone by, would I still be wanting it?

What is the thing? What functionality considerations do you need to make? What does it need to do? If you already had it - what would it be doing? Will it fit in your life?
What is your expected use? Daily? Once-off? Occasional? (no more than 5 times in your predicted future)
What do I want it to do? Does this thing do what I want it to do? (It can be very easy to buy a thing that doesn't quite suit the need because we get distracted between wanting a thing and getting a thing)

Consider your options that avoid buying it:

Can I borrow one from a friend? Or a family member? (some things cannot be borrowed like a wristwatch - no sense borrowing one if it’s an item you wear every day - or other reasons to not borrow a thing)
Can I get one second hand?
Some items are perfectly fine second hand, i.e. books, whereas others are potentially less fine (i.e. cars) where more can go wrong with a second hand one. The point of this inclusion was to encourage you to consider it when you previously would not have. for whichever reason. Books second hand can also be occasionally out of date or damaged; and cars second hand can be excellent purchases.
Is anyone I know also interested in having the thing, and would they be willing to split the cost with me in order to have it on a kind of timeshare, and can we agree on a deprecation schedule such that one of us buys out the other's share in the future, if one of us is moving away or something?
Renting/hiring the thing - as a one off. (works for most power tools, as well as storage space, a boat, all kinds of things...). It is also an option to rent short term while you decide if the thing fits your life. i.e. rent a jetski. If you find you don’t use it enough to warrant a full purchase you only needed to invest a little bit of the final cost; and might be saving money to do so.
Timeshare - businesses exist around sharing cars; boats; holiday houses and various other products. You might be able to take advantage of these businesses.
Can I apply for credit for the thing? Can I get the item on consignment?
Could I earn money using the thing and return some costs? (Am I likely to do that based on my past experiences doing so with other purchases?)

Knowledge about the thing:

Do any of your trusted friends have opinions or knowledge in the area?
What do online reviews say?
Is there a community of enthusiasts (i.e. Online) who have resources or who you can outsource the search to?
Are there experts in the field - (i.e. buying houses), is it worth engaging an expert for this transaction?
How much time do I want to spend on considering and shopping vs how much use will I get out of the thing? (for items under $20, try not to spend more than half an hour on it; or it’s almost better to randomly buy one available {depending on your local minimum wage})

Purchase considerations:

What is my budget?
Can I afford it? (see options that avoid buying it)
Price range of the things on the market?
Is it cheaper somewhere else in the world and posted to me?
Can I ask for a discount?
Can I combine postage with other items?
How long will the thing last?
How long do I need it for?
How quickly do I need it?
Do I want to be able to sell it when I'm done?
What's the return policy of the various places selling it vs price vs shipping?
What is the shipping time?
Does it come with a warranty? Does the warranty last long enough for my liking?
Are any laws, customs or taxes applicable to it; or its purchase, or resale?
What's the difference between the best price and the worst price, and when do you wind up spending more time (in terms of the value of your time) than that difference trying to get the best price?
Does it have resale value? Do some have better resale than others? (are you actually a person who re-sells things? - have you resold a thing before?)
Can I get it in a physical store? Can I get it online?

General specifics:

Is the one I want a quality item?
Is the item disposable or not? Have you considered the merits of a similar but disposable one? (or a similar non-disposable one)
Does it have the correct colour? Or other embellishments?
Do I have storage space for it within my existing storage area?
Is it big? Can I get a smaller one?
Is it heavy? Can I get a more lightweight version?
What are its power options? AC, DC, battery, built-in battery, built-in solar, etc.
What is it made out of? Does it come in metal, plastic, wood, etc. what would I prefer?
Does it suit my existing possessions?
Will this one cost more to repair than the other similar ones?

Miscellaneous considerations:

Do I have a backup for if this one fails?
What are the consequences of a lower-quality thing breaking while I'm using it?
Can I pay for it from someone who is going to donate proceeds to charitable causes?
For any purchase under $50 (adjust for your life circumstances) it’s not so much worth running through this checklist; but for more expensive purchases - it’s likely that if you want to appreciate that you put in effort and came to a good conclusion, a process like this will be helpful.
Is the process of buying it give me pleasure? Or I will suffer in a long line for it?
What kind of signalling is the thing going to give me? Do I want that?
Does the thing have an upkeep or maintenance cost?

Nearly all of the points listed here could be expanded to its own post. These points apply to everyone to different extents. “Considering borrowing” is advice that is priceless to one person, and useless to another person. similarly; “budget” might be significant to one person because they don’t spend often but then spend whatever they like when they need to; but useless to another person because they live and breathe budget.

I plan to cover this in another post about making advice applicable to you.

meta: 3 hours write up. 3-5 reviewers, slack channel inspiring the post, and giving me a place to flesh out the thoughts.

This post is certainly open to improvements. Please add your comments below.

See also: My Table of contents for other posts in this collection.

See also other repositories on lesswrong:

Low Hanging fruit for buying a better life - What can I purchase with $100 that will be the best thing I can buy to make my life better?
Financial Effectiveness Repository - Recommendations and ideas about financial effectiveness?
Boring Advice Repository - Also has quite a lot advice on how and what to buy.