Garage sale hosts don't usually offer return policies for a reason: it opens up way too many scam opportunities. But not only that, it also allows customers to bother the host if they don't know how to use the item or are searching for technological help. If you decide it's a good idea and a policy that will help you sell off a lot of high-priced technology, offering technical support is an option for you as a host. Just make sure that you know what you're doing!
Only certain items should be eligible for your technical support. First of all, the items should be out of the scope of their original technical support, meaning that the company that produced them should no longer allow customers to call on those particular items. Second, you should know the item inside and out, so that if a problem occurs, you'll know exactly what to do. Third, they have to be items you are sure work.
How do you let a customer get in touch with you for technical support? You'll have to give them a way to contact you. Phone is good, because it speeds up the efficiency with which you find out what's wrong with the item. Email is also useful, because then you can ignore it until you're ready to address the problem. Try not to ask them to come directly to your house, as you don't want people showing up on your doorstep with your old items!
If the item is something that is difficult to transport, then you'll need to have a limit to the distance you'll travel. Don't sell something to a customer who lives out of state and then promise technical support; make sure you know where the item is headed, if you expect them to summon you should something go wrong. Also, be sure you get compensated for distance traveled; unless the house is a block away and you can walk there, it will cost you gas money and wear on your vehicle to get there.
Decide if you'll offer technical support as an additional paid service, or for free. You'll need to decide on this ahead of time, so you aren't left trying to make that decision in the moment. It may depend on the item for you, and if that's true, clearly label the items that are free to distinguish them from the ones you'll charge for.
A good garage sale host would list their sale on GarageSaleCow.com, the nation's biggest garage sale website!