Many of us love a garage or yard sale. It was once estimated that there were about 165,000 such sales every single week in this country, and while the rise of online marketplaces may have cut into those numbers in recent years, the old-school garage sale still has some advantages. You don’t have to deal with shipping or arranging pickups, for example, and you might clear out your entire unwanted inventory in one hectic day instead of spending the next few months of your life selling things one item at a time.

There is one big downside to a garage sale, however: vultures.

Garage sale vultures are people who bring a ruthless professionalism to shopping garage and yard sales. They’re often resellers who identify stuff of real value, buy it cheap from unsuspecting people hosting a sale in their yard, and then resell it for a big profit online. They may also be collectors who know that rare video games, comic books, or other items are often sold for pennies at garage sales.

There’s nothing illegal about being a garage sale vulture, but they go against the spirit of a yard sale, which is usually a friendly, casual affair. It’s useful to be able to identify them, however; when a vulture invades your garage it usually means you have something worth more than you think—maybe a lot more. If you spot a vulture, you might want to rethink your pricing strategy.

They’re weirdly aggressive

If you’ve ever organized a garage sale or let your kids set up a few tables outside the house in an exercise of junior capitalism, you know they tend to be sleepy events. If you advertise a little, you might get a few folks from around the neighborhood, but everything’s usually pretty calm.

Vultures tend to harsh that vibe. The day of the sale, they show up early and wait impatiently outside your house. They jump gates and can be very pushy because they’re worried someone else is going to figure out that you’ve got valuable merchandise and scoop it up before you can.

They bring professional equipment

Most of the people who show up to browse your garage sale will be neighbors and curious passers-by. If they buy something, it’ll be based mostly on vibes, or because they spot something with specific meaning to them. Those sales are a win-win—everyone is happy.

Vultures aren’t there for the vibes. One sure sign you’re dealing with one is that they brought professional equipment to assess your offerings—a jeweler’s loupe to look at what you thought was your grandmother’s costume jewelry, or reference books to look up the valuations of your ancient baseball cards. They came for the profit margin, not the thrill of finding something cool for cheap.

They ask for previews

If you’re ambitious and advertise your sale ahead of time, online, or via the tried-and-true “flyers stapled around town” method, vultures may pore over any information you post and contact you ahead of time. They may call days before the sale and ask if they can come look at something you mentioned in your ad or that they spied in a photo you posted. They know something is valuable, and they want to try to get their hands on it before someone else carries it off—or clues you in. Being contacted before the sale even begins is a clear sign you’re being targeted by a vulture.

They buy in bulk

Collectors and resellers often try to save time by simply buying your entire inventory of something for a low price—boxes full of old comics or unsorted jewelry, or that entire plastic tray of ancient video game cartridges. The offer is often presented as a convenience for you, as they’ll take everything off your hands for a few bucks. This spares them the trouble of determining if any of it is actually worth something right then and there. They can toss your stuff into the garbage later if nothing pans out.

Your purpose in hosting a garage sale is to get rid of junk you don’t want and make a few bucks in the process, so selling something to a vulture may not matter to you. But vultures can make a fun day less pleasant if they’re aggressive, and their presence often signals that you have something worth some real cash, so being able to spot them before they invade your yard is a useful skill.