A trusted customer came into the shop the other day with a fixed gear conversion. He purchased said bike from a "bargain" bike seller who shall remain nameless. I don't recall how much he paid for the bike but it included the frame/fork, BB, crankset, handlebars (flopped and chopped), and a wheelset with a flip-flop hub.
Several things struck me as out of the ordinary: the axle on the rear wheel had some extra nuts that didn't match in width to each other. Most of us know that a flip-flop hub has a 120mm spacing. Older model road bikes have a 126mm rear spacing. Simple math tells us this is a difference of 6mm, which is 3mm on each side. It is perfectly fine to tighten the axle nuts that 3mm. You won't mess up the frame. If you add extra nuts you are in effect pushing the cog further away which will result in a off chainline. Of course that chainline also depends on the BB length and crankset you are using. Every crankset sold now will have a recommended BB length for its intended application. Most fixed gear cranksets run anywhere from a 102mm to a 110mm, while road bikes can run from 110mm to 122mm. I don't know what the BB length was on this bike because I didn't measure it, but let's guess it was a 117mm. Obviously you need to run your chainring on the inside of the spider to get your chainline as straight as possible, which brings up the other item about this bike that I found odd: the chainring was run on the inside of the spider but the chainring bolts were intened for a double chainring. Obviously that means they are longer. Instead of using chainring bolt spacers this "bargain" bike seller used washers. Honestly, that will work fine provided the washers are small enough so they don't hit the spider.
These things combined (mis-spaced rear hub, longer BB) will result in a chainline that is way off. What happens when your chainline is off? Unnecessary wear on the chain, chainring and cog and your chain could pop off or break resulting in serious injury or even death.
If your bike looks like this you should take it to a qualified mechanic at one of your local bicycle shops and have them fix the problem or take it back to the seller and get your money back. I wouldn't ride it and no shop would sell it.