minimum purchase and sell rules

to my knowledge when purchasing shares you must purchase a minimum of $500. Whats the deal with selling? In regards to mdmoc (medical monitors options 20c June 05) I had an order a 2 weeks ago to purchase 400,000 @ 0.004 and was hit with a sell of 2548($10.19 worth) thus giving me a measily $10 worth and being hit with $29.95 brokerage for my incomplete order. today someone was hit at 0.008 with 1375 sold($11 worth).

To me this ploy has been to make the option price look weak after someone made a purchase at 0.012 thus bringing it into line with the FPO's shares price move recently of about 30%.

why would someone sell such a small holding, I don't believe it was to complete a incomplete sell order so even though you must purchase $500 worth or shares you must be allowed to sell $10 worth. correct?