When to Sell

It is rarely a good idea to sell a property.

Selling incurs capital gains tax, so you lose half of your profits. A better approach is usually to refinance the loan.

If you bought a property for $100,000 with an 80% loan ($80,000) and the property goes up to $150,000 you can sell it, get a $50,000 profit, pay up to $25,000 in tax and pocket $25,000.

Alternatively, you can refinance it to 80% of $150,000 or $120,000. This gives you an extra $40,000 in your pocket (from the bank) plus you get to keep the property.

Despite this, if you own a property that is costing you a lot to maintain and which is attracting poor tenants who constantly give you more things to maintain (breakage) it might be better to get rid of it.

Edit: Don't sell a property because of poor tenants - change the tenants.
If, however, the property ONLY attracts poor tenants (it suits that end of the market), perhaps it would be better to sell the property.