What is a seller’s liability regarding a property after the sale?

If the buyer purchased the property without requesting that the seller place handrails on the staircase before closing, then after sale that is the buyers call.  The property is sold as is.  If the buyer accepted the $500.00 the seller offered not to disclose hidden hazards, then it is on the buyer.  Besides a handrail is not hidden, so if the buyer bought the house like that and he did not request the handrail to be placed before closing, then he purchased as is and the seller is no longer responsible, unless at the closing the seller promised to fix the defect in a reasonable time.

It is highly unlikely that you could hold the seller responsible for an "apparent defect" (if that constitutes a defect) like you are describing.  Think of it this way, if there was a huge hole in the floor (like the size of a bathtub) and you bought the home and then you fell in the hole, you would be hardpressed to blame the seller for falling in the hole.  Sure, you could have reasonably demanded he fix the hole before the sale, but you did not.  This situation is not all together that different.  You might be better off looking at your home owner's policy for a situation like this.  It is, with an "open and obvious" issue as you are describing harshly, "Your house, your problem."  I am in agreement with MTD above.  Note that the result could be different if the defect were "hidden", but a missing stair rail isn't what one would likely conclude to be hidden. 

Possibly, depending on the nature and extent of the disclosures made prior to the sale.  It may also depends on what the buyer knew/agree-to-accept when the purchase was completed.  Also, it may matter who the injured party was, i.e., was it the new owner who arguably knew of the condition, or a guest who arguably did not.

When you sold the home, you sold the home as is, and it is up to the buyer to have the home inspected and it is up to the seller to indicate the blatant defects in the house that can pose a safety issue. These blatant issues should most likely be listed in any purchase and sale agreements so that everyone has actual notice of the issue. Further, the inspection should list those obvious issues. If the buyer's broker comes up and asks for you to fix it prior to closing, then it would be up to you to do so and you can theoretically be held accountable for any injuries incurred if you do not fix it prior to closing.