My first thought was: Don't let them do that. Don't make magic plusses something that wizards can control. Limit in-game descriptions of magic swords to:
- It's magic;
- It's Lawful or Chaotic;
- It slays lycanthropes (or giants, etc.) or has one specific power.
Roll additional powers for the sword after the wizard enchants it; don't let the wizard or the buyer control this, or control the plusses. Roll the plusses randomly.
What you could do is make a reaction roll. A Bad reaction means a cursed weapon; Average or Good means +1, Very Good means +2. Magic weapons can't be made from common metals, but must be made of meteoric iron, silver, etc. Mithril adds +2 to the roll, while adamantium adds double. On 2d6 reaction rolls, a 13-14 is +3 and a 15+ is +4.