Adults are scooping up promotional items out of nostalgia - and playing into a frenzy around collectibles

Starbucks launched its "Peanuts"-themed fall collection on Tuesday.

Customers this week snapped up limited-time gear from Starbucks's "Peanuts" fall collection and McDonald's new "SpongeBob SquarePants" toys - and now they're being resold online for a premium.

After Starbucks (SBUX) launched its collection of glass Snoopy cold cups and other gear featuring Charlie Brown and Linus on Tuesday, the items had largely sold out on its website. The Snoopy cup was going for as much as $120 on Etsy (ETSY), compared with the $40 price listed on Starbucks's website.

As for the McDonald's (MCD) "SpongeBob" Happy Meal toy line that also launched on Tuesday, some eBay $(EBAY)$ merchants were selling the entire set of those toys for around $90. And a set of "Dragon Ball Super" figurines launched by Burger King $(QSR)$ this month, along with cups and a paper crown, was selling for more than $40.

Of course, other merchants were selling those items for lower prices or had recently slashed their prices, and prices can be negotiated downward. But the demand and price swings have arrived amid a booming market for collectibles, with people increasingly treating items like trading cards and toys as investments.

Neil Saunders, a retail analyst at GlobalData, said the toys and other gear offered by restaurants particularly appeal to that market given that they often arrive in the form of limited drops. Other factors also add a sense of scarcity.

"You have to be a member of a particular tier in a loyalty scheme or make a particular purchase to get them," he said. "Because of that, it makes them desirable and interesting. And of course, once it goes on an auction site like eBay, it's very easy for that price to go up."

Happy Meals and other toy promos were geared toward children in the past. But Saunders noted that they're now more likely to be geared toward adults, who might be lured by nostalgia or the collectibles frenzy. Additionally, social media has made it easier for items to go viral, and brands have become more savvy about what might resonate with consumers.

A Starbucks spokesperson told MarketWatch the company increasingly sees its ancillary offerings - like cups and other gear - as collectibles. The company declined to offer sales figures on the "Peanuts" items or details on the exact percentage of stores selling them, nor did it say whether it had seen any unusual buying activity surrounding the items.

McDonald's did not immediately respond to a request for comment. Neither did Burger King parent Restaurant Brands International.

The Starbucks items featuring the "Peanuts" characters - known for animated classics like "It's the Great Pumpkin, Charlie Brown" - were intended to promote the return of Starbucks's pumpkin-spice latte. The move follows the coffee chain's other recent collaborations with Hello Kitty, the clothing and accessories brand Roller Rabbit and the fashion brand Dandy. Those efforts have been part of the company's turnaround plan, which has shown signs recently of taking hold.

Earlier this year, limited-edition Imax (IMAX) popcorn buckets, designed to look like the Imax camera Christopher Nolan used to film "The Odyssey," began popping up on eBay at inflated prices.

Saunders also noted that a Dunkin' promo this year offering a free tote bag with the purchase of a half-dozen doughnuts went over well with customers. And he called out the enduring appeal of Trader Joe's tote bags.

"They've become super popular, to the extent that they've been popping up in Japan and the U.K., where there are no Trader Joe's," he said. "Why are people interested in this basic tote bag? I think it's because it just becomes a viral sensation online, and it becomes a real collectible thing, and it becomes a must-have item."

-Bill Peters