Technically common property isn't sold to a person, they are granted exclusive use of it. To do that the owners corporation needs to be approve a special by-law, and that has to be by way of a special resolution. Ordinarily the by-law will have conditions attached to it relating to the person who has exclusive use being responsible for its maintenance etc.
The by-law also has to be registered. Was a by-law passed by special resolution and if so, was it registered? I would check the minutes as a matter of urgency. You should also check the financials to see if the $12,000 was paid. If there was/is no by-law then query whether the person has the right to exclusive use of that area.
Where someone is being given exclusive use of a part of the property usually a valuation is done, taking the improved value and deducting the cost of any improvements the person has done to it – the difference should be paid to the owners corporation. This is because the common property is owned by all the owners and therefore they should be compensated for no longer having use of/entitlement to the area.
The whole thing does sound dodgy, but I think there was a quorum because the treasurer had the proxies. The question then is was the meeting properly convened, was there a special resolution etc? The other strange thing is on the strata plan you had 2 years ago, this was showing as common property and you bought on that basis, so what is the strata plan showing now? Was that strata plan provided to you by the vendor, because you could also have a right of action against them.
This is the section of the Act dealing with exclusive use.
52 How does an owners corporation make, amend or repeal by-laws conferring certain rights or privileges?
(1) An owners corporation may make, amend or repeal a by-law to which this Division applies, but only:
(a) with the written consent of the owner or owners of the lot or lots concerned and, in the case of a strata leasehold scheme, the lessor of the scheme, and
(b) in accordance with a special resolution.
(2) A by-law to which this Division applies may be made even though the person on whom the right of exclusive use and enjoyment or the special privileges are to be conferred had that exclusive use or enjoyment or enjoyed those special privileges before the making of the by-law.
(3) After 2 years from the making, or purported making, of a by-law to which this Division applies, it is conclusively presumed that all conditions and preliminary steps precedent to the making of the by-law were complied with and performed.