For years now, I have been able to make a little extra money by selling my family's used books, movies and video games online, but lately that's become more difficult.

The problem: robots.

They're faster than me, and they never sleep. If I list a book or game for sale online — bing — a rival seller quickly underprices me, by exactly a penny.

This type of constant price-cutting is a good thing for people who buy used goods online, but it's a real headache for individuals who want to sell things. I'd like to be able to list an item for sale and forget about it until someone buys it, but that's hard to do when the robots are constantly undercutting my prices.

The robots are the systems used by large resellers who, like myself and other individuals, sell books and things on websites such as Half.com. They constantly search the Internet, collecting information on what people are buying and selling, and for how much, and they keep updating prices in order to be the lowest and boost sales volume.

Here's how this works: Let's say you want to buy the book “Life of Pi” or a DVD set of the first season of the HBO series “Game of Thrones.” You could go to Half.com and see that copies of “Life of Pi” are offered at a range of prices, from about $9 for a new copy to around $3 for a used one in good condition, so you'll probably choose the condition that's acceptable to you and then buy a copy in that condition with the lowest listed price.

So if you're trying to sell your copy, and other sellers keep underpricing you by a penny, it's going to take longer to sell yours, and you're going to have to keep going back online to update your price. Otherwise, you'll never have the least expensive copy in a certain condition, and why would someone choose the second or third-lowest price?

Of course, this is all good news for thrifty buyers. For example, this week you could save about $10 purchasing a new set of the first season of “Game of Thrones” on DVD from a reseller, and you can assume prices on any relatively new book, DVD or video game will fall considerably over the course of a few months.

Prices on newer items fall quickly because people buy them, then read or watch or play them, and then resell them, adding to the supply.

Individuals like myself sell used books and things online because the alternative is to take the item to a business that will pay substantially less than the going price, and then they will sell the item.

For example, I just sold one of my son's used video games online for $26 plus shipping. That involved listing the game online, packing it and going to the post office to ship it. Another choice would have been to sell it to a reseller like Mount Pleasant-based AbundaTrade, or we could have gone to the shopping center and sold it to GameStop, where I was told the game would be worth $16 cash or $26 in store credit with a promotional deal they are running.

Popular video games fetch good prices, but used books are another story. If you're not a high-volume reseller, is it worth your time to try and get $3 for a book? Listing it online is no big deal, but having to wait in line at the post office — and you do have to wait in line to ship things at cheap “media mail” rates — is a time-consuming errand.

One tactic I've been testing out is, when I sell something online and know I'll have to go to the post office, I've been deeply discounting the prices on my remaining items in one-day-only sales. I suspect the price-cutting robots won't drop their prices below my unusually low prices, and I'm already going to the post office, so any additional sales the same day would be a bonus for me. We'll see how that works out.

The other choice with books is to go to a local reseller, such as Mr. K's in North Charleston, where you can get store credit or a small amount of money if they want your books.

I haven't yet given up selling things online, but between the price-cutting robots and the lines at the post office, that day may be coming.

Reach David Slade at 937-5552, dslade@postand courier.com