Well,what can I say,but
If you buy a cheap speaker pair and a amp some sort,You usually CAN do your mixes on this.But youll have problems getting a even sound.So you need to get to learn your speakers,the best way is to testdrive your mixes on alot better systems and try to remember what goes missin and what you have to much of.
It is quite possible,but it takes time,and your mixes will not be wery good at a start.A good thing is that you will get alot of experience and learn alot about mixing(the hard way).
In combo with a good pair of headphones youll be pretty well off after a while.
Ive been doeing this for years on a pair of pawnshop speakers
and my mastermix studioguy is usually pretty happy with the results.
A tip when buying speakers in thriftstores and pawnshops:
Knock on the cabinet and check so that it sounds VERY solid otherwise dont buy.Also check the binding posts and make sure these are of a high quality.
Remember - Make Music !