Q:

What to look for when buying a light microscope?

I am a science lover and want to buy a microscope to explore things around me. Like studying cell structures, microorganisms, blood and plants. It would be great if there is way to photograph slides. Just curious to know and explore things around me.
I want to know which features I should consider while buying it?

A:

Well it depends on the application. First you have to determine what it is that you want to view under the microscope e.g. an organism such as Drosophila or you want to see microscope slides? In the first case you need to by a dissecting microscope, otherwise you need to by an upright light microscope with the appropriate objective lenses, again depending on the application. If you are viewing cells in a flask/dish/plate in a cell culture setting or if viewing a haemocytometer (although not necessarily in the case of haemocytometer), you need to buy an inverted light microscope with phase contrast since you wouldn't be able to view the cells very well with an upright microscope since the goal is to reduce the distance between the objective and the sample for a better view of the samples in cases of dishes or plates. Again the choice of equipment depends on the application. But as correctly said above you need to clarify what you mean in the question.

A:

My advice is always the same: "first microscope? keep it simple, keep it cheap, but nevertheless: buy something usable". 
This Olympus GB is, IMHO, "something simple/cheap/usable": 

These can be bought here second-hand for anything between € 50 and € 100. It's an uncomplicated stand, easy to overhaull yourself. It permits the use of any optics you would like, it has a nice Abbé-condenser N.A. 1.25. It's heavy,  sturdy and reliable.
These are some of the microscopes I bought second-hand over the last few years. None of those was more than 80 Euro's/US$, except the Diapan (lower right). All were in reasonable condition and ready to use, except one. Notice the build-IN 6V-15W illuminator in the Zeiss GF stand (lower middle), versus the build-ON 12V-100W HAL illuminator in the Reichert Diapan (lower right). See the text:

And if you're handy and not affraid to work it can be even cheaper. This one was for free and it turned out to be one of the best stands I ever had:

Perhaps this can help: it's a (partial) checklist on what to look for when buying a second-hand microscope.
I don't have absolute "no-no's": many problems can be solved, but sometimes it takes a lot of effort, time and cursing... 
Sometimes a stand is junk, but it has excellent optics. Or the other way around. 
You should take into account that having a microscope (seriously!) serviced/repaired is usually very expensive: a company here, known for their high quality work and their very reasonable prices, asks for a complete overhaul of a microscope genre Zeiss Standard 14/16 a fixed price of slightly more than 700 Euro, spare parts not included... On request they do it on an hourly basis as well: 120 Euro/hour. They do an excellent job, but well... it's not cheap.
1. A first glimpse
Brief examination of a second-hand microscope would take half an hour, up to an hour, depending on how sophisticated it is and on how experienced you are. 
You're allowed to be exited and thrilled, but don't loose your critical sense! It's only a microscope. Plenty of those and there are far more important things in life.

Microscope complete, all parts present, all same brand and same
generation (do your homework!). All knobs, screws, gears... present as far as one can see 
Suitcase present and in reasonable condition, key present if one is needed, factory certificate present, user manual present
Microscope reasonably clean, no excessive dirt, no severe traces of
corrosion or fungus (if fungus is present, even in very small
amounts: carefully check all optics!). No excessive traces of extremely heavy use. Check aperture diaphragma dial, field diaphragma dial, surface of the stage, x- and y- dials of the stage if present, the surroundings of the focusing knobs, the nosepiece... 

Spider webs and dirt on a vintage microscope, stored for decades on an attic. Turned out fine: 

Microscope doesn't spread strange smells: fungus, dish soap, cleaning naphta, white spirit, xylene, ethyl alcohol, hexane, greases and/or oils, "creeping oil" such as WD40 or CRC... A microscopes "treated" with creeping oil is about the only absolute no-no I have: these oils do what they're supposed to do: creeping in everywhere, including in the optics
Microscope doesn't show any trace of a fall or a punch. If that kind of damage is present, decent repair should be demonstrated beyond any doubt by official documents only, excluding hearsay: repair costs estimates, invoices, correspondence between owner and insurance company if applicable etc... (observe serial numbers in those documents. They're there for a reason!) 

Damage due to a drop in a Zeiss Standard microscope. A cleaning lady dropped it accidently, while cleaning a college lab. Accident and repair were well documented. Replacing it would have been less expensive, but the insurance company insisted on a repair, which was done in the Zeiss factory. Nearly all parts were replaced, except for the base of the stand, as that was considered "cosmetical damage" only. The microscope was equipped with high end optics and the price was modest. A steal: 

All adjustments (coarse and fine focussing, nosepiece, condenser height adjustment, aperture diaphragma, field diaphragma, movements of the stage, eventualy present internal magnification changers, Bertrand lenses, polarizers, ... work well over their entire range. Use your senses: sight, hearing and, most important: your tactile sense. Be very aware of "strange things" when using the settings: scratching noises/feel, ticking noises/feel and so on. Might be a minor problem, such as a small lump of dried out grease. Or might be a broken/stripped tooth gear... Very stiff controls, even up to the point of being completely "frozen up" are usually not that big a problem. However, keep in mind it might be necessary to dismantle the microscope completely to solve the problem. That means: dismanteling, cleaning and degraising, regreasing using the right lubricants in the right amounts at the right places, reassembeling and alligning the stand. It's not rocket science, but it is a lot of work! Pay attention to the leafs of iris diaphragms: these should always be dry, and free of grease or oil 

Dismanteled coarse and fine focusing controls of a Zeiss Standard. Notice the ingenious construction using a planetary gear:

2. A closer look
3. Microscope under the microscope
Second-hand microscope buyer's best friend: a phase telescope:

"Minor" optics problems...
Seeped in immersion oil and degraded lens kit in a Hensoldt 100/1.30 achromat (awaiting a go for repair, succes unlikely):

"Small scratch" and extensive delamination in an Aus Jena apochromat 40/0.95 (garbage bin):

Unsetting fungal growth in an Olympus 10/0.25 achromat (repaired):

4. Using it
Counting dots/objective testing using test slides: I'm no fan:

Continues: work in progress...

