What criteria do I need to be aware of when buying a 3D printer for personal use?
Here are few things to consider from my point of view



Printing technology

The first thing that you need to take into account is printing technology. The most common[citation needed] right now is Fused Filament Fabrication. "Liquid light-sensitive resin" is being used in Stereolitography and Digital Light Processing - the SLA printers I found are less common and more expensive than FFF ones.



Price

Need to decide on budget. You can buy printer for 60k USD and 400 USD. Quality is somehow linked to price but that's not a rule. You can buy a shitty printer for a lot of money.



Printing area

Bigger allows you to print bigger things. You need to ask yourself how big things you really want to print. Remember that 3d printing is quite slow process - how often you will want to print big things that will take 60hrs+ to finish?



Printing materials

What kind of materials you want to print with? Some materials will need higher temperatures so check the max hot-end temperature, some will require heated bed.



Assembled or DIY kit

You can usually get kits for self-assembly cheaper than Ready-To-Print machines. However, it will require additional skills (i.e. soldering), tools and time to assemble. I am not sure if I would buy DIY kit for commercial use, but as an enthusiast I immensely enjoyed putting my Rostock Max together.



Reviews and reputation

It is generally safe to buy printer that already has some users. Beware of new magical Kickstarter printers which will "change the 3d printing forever".
Reddit /r/3dprinting suggests that your new printer should meet 3 criteria:


Printer passes the youtube test - has lots of youtube evidence that this particular printer is working.
Printer is out of the pre-order phase. This means that all pre-orders have been delivered.
Printer has a reputation of working well among current users.


I found it to be a very good set of rules.



Upgrade capabilities

That's very user-dependent, but this point is very important to me. I want to be able to change and improve certain parts of my printer. Check if you can switch the extruder, replace the hot-end etc. 



Support

I think one of the most important points. See if you can find a forum for your printer and how active community is. It will be immensely helpful if something goes wrong (and it will).
Also, company support is very important. What will happen if you need a replacement part or your printer will stop working altogether?



This list is definitely not complete.
There are many more things that might be taken into account like configuration (delta or XY), multiple extruders, closed cases etc.