Even before Apple sent invitations for the Tuesday launch event where it will likely unveil a smaller iPad, consumers were unloading their old tablets in increasing numbers so they can trade up to Apple's latest, according to gadget buy-back firm Gazelle.

Today, rivals of the Boston-based company said that trade-in volume continued to climb, with owners of 7-in. tablets from the likes of Google and Amazon joining the bandwagon. (See: Live feed: Apple's iPad mini launch event)

A week and a half ago, Anthony Scarsella, chief gadget officer at Gazelle, noted that tablet trade-in volume was up 80% in the previous 30 days over the same stretch before that.

"Volumes started going up in mid-September, when they doubled in one day," said Scarsella. "Since then, they've stayed pretty stable."

Although rumors of a smaller iPad -- usually pegged as a tablet with a 7.85-in. display, considerably smaller in total area than the 9.7-in. iPad -- have circulated for years, talk in mid-September centered around the tablet hitting production at a major Apple supplier.

Follow-up speculation a few weeks later tapped Oct. 17 as a launch date, with a Nov. 2 on-sale debut. Those rumors did not pan out; last week, Apple invited reporters and analysts to an event tomorrow, Tuesday, Oct. 23, where virtually everyone expects it to reveal the small "iPad Mini" -- the name most have used as its until-launch label.

"Most of the tablets being traded in are original first-generation iPads," said Scarsella, adding that 60% of the requests for quotes were from users seeking a price on the 2010 model.

"It looks like upgraders want to turn in their first-generation iPads," Scarsella. "I assume that they're looking for a more updated model, since the presumed specs [for the iPad Mini] are similar to an iPad 2."

As of 4 p.m. ET today, Gazelle was quoting $125 for a 16GB first-generation, Wi-Fi iPad in "good" condition, $130 for a 32GB model and $140 for a 64GB device.

Although iPad Mini prices have not yet been revealed -- Apple has not even confirmed that Tuesday's event will introduce a smaller iPad -- bets by experts and pundits have ranged from $249 to $329 for the entry-level configuration.

Other trade-in vendors have seen activity jump as well.

NextWorth's current quote for a first-generation, 16GB iPad is $130, about half the lowest predicted price of an 'iPad Mini.'

Last week, NextWorth said that iPad quotes increased 85% over the previous 30 days. NextWorth today said it would pay $130 for a 16GB first-generation, Wi-Fi iPad.

At Glyde.com, a company founded in 2009 by several former eBay executives, iPad listings were up 10% today versus last week. Like Gazelle, Glyde said older models of Apple's tablet accounted for the majority of activity.

Glyde's business model is different than Gazelle's or NextWorth's, which buy electronics devices -- mostly smartphones and tablets -- from consumers then resell them on Amazon and eBay, or to wholesalers. Instead, Glyde serves as an intermediary between sellers and buyers, and takes a cut of all sales.

Glyde's suggested selling price for a 16GB first-generation, Wi-Fi iPad today was $200, with the seller getting $176.50 of that.

Other tablet owners are listing their hardware on the site, too, said David Pederson, a spokesman for the company. "Inquiries for all other tablets -- Kindles, Nexus 7, Nook, and so on -- were up 35% [last week] versus the week before," said Pederson in an email reply to questions. "Google's Nexus 7 saw the biggest gains with a 300% increase in inquiries."

The Nexus 7, which Google rolled out last June, is a 7-in. device priced at $199.

Nexus 7 owners, however, may not be selling their tablets to buy an iPad Mini, but in the expectation that Google will take the wraps off new models of the diminutive tablet, as well as a larger 10-in. device reportedly made by Samsung.

The escalation in trade-in activity prior to Tuesday's event has been much less pronounced than seven months ago, when owners stampeded to sites like Gazelle and NextWorth to dump their older iPads before the launch of the Retina-equipped "new" iPad, called "iPad 3" by many, if not Apple.

Then, Gazelle reported a 500% increase in quote requests compared to the month before.

See also:

Big thoughts on a little iPad

iPad mini could be 'competition's worst nightmare' says analyst

Apple expected to drop iPad 2 upon iPad mini's arrival

iPad mini to be called iPad Air and launch without Retina display?