Merchants by any means are all around good people when it comes to making a profit (selling or buying otherwise) and will gladly provide help to any who wish to seek their services.
Merchants are players who both buy and sell items. They are generally peaceful and stick strictly to their business. No matter what, though, their job is to make a profit. Don't be expecting to get anything for a discounted price, though. NPC merchants, though rarely seen in around SAO, are programmed to automatically take in and sell items when appropriate. Player vendors will usually stick to selling items for higher prices and buying rarer weaponry and equipment.
Weapon Appraisal being used to determine a weapon's value.
Preferably, merchants stick to the towns or "Safe zones" and prowl around for anything good to either take or grab and sell it later. After all, money is the most valuable resource in SAO. Prices for the items are not set in stone, instead they are randomly generated by The Cardinal System. To keep the generation of money flowing throughout the system, prices are usually set to lower standards based upon the stats provided for the weapon, armor, or equipment.
Necessary skills, such as using Weapon Appraisal, will likely generate a positive influence of currency to those who use it to their advantage. Certain techniques vary greatly amongst the users of the skill and will vary upon accuracy. Practice makes perfect with this skill.
Purchasing every weapon on the streets might not be a great idea, as some weapons could be for no luck in value. Depending on the situation, application of the Weapon Appraisal skill might not be a sufficient way of looking for the value. A good merchant knows what people want to get from him/her and will get those desired items if the time permits such an exchange. Not all merchants are good. Some will use dirty tricks and thievery to repeatably take back the items that were sold so they can be sold all over again.
While true that valuable items with higher stats are good to have, nothing can come close to the weapons that have been a part of the player's inventory since the very beginning. Bigger is not always better when it comes to the blade that the sword wields, or the player who uses it.
These systems of merchants is a valuable resource to have when considering how well the market system plays out. Vendors, towns, cities, and other spots of interest for market greatly increase the sphere of influence that they have over the general population of players.