First you must understand that it is 'ILLEGAL' to sell and buy Residence Visas in Qatar. It is a serious crime.

If caught could lead to jail and depratation with life ban...

If someone succeed, then as Mr.Qatar said there are high chance of getting conflicts with the sponsor to transfer visa to another sponsor (most of them the ask for money again).

Depends on the nature of job you find in Qatar becasue earning QR. 35,000 for labourer, low and middle class income is very very difficult. You need years to recover your money.

Also you must take all the factors in your mind, such as cost of living, local culture, social life, medical facilities etc???