It's an offensive assumption, but it's common. It's even more offensive when it comes from someone who's financially privileged, and has no idea what it's like to live on a very limited budget.

Pointing to things like computers and televisions as either reasons that a person isn't poor ("you're not poor if you can afford unnecessary stuff"), reasons that a person is poor ("you wouldn't be poor if you didn't spend your money on unnecessary stuff"). or reasons that a person doesn't deserve assistance aimed at the poor ("you don't deserve my tax dollars paying for your dinner until you pawn your unnecessary stuff") -

They're all ways that privileged people blame the poor. You're going to touch a nerve if you echo those attitudes.

Plus, as has already been mentioned, you have no idea what it's like to be poor. That's not really your fault. You're privileged, your aware of it, and you're trying to learn. *clap clap*

You can get computers for pretty cheap these days. Poor people can save up, buy a cheap computer, and then keep it far longer than people with more money would. Selling it would only bring in a few bucks; it won't make a damn difference in the long run, and those few bucks would come at the cost of cheap entertainment (should poor people just stare at the walls?), keeping in touch with family and friends, your kids' education, etc.