Thinking of selling

I’m thinking of starting off small by selling under $100 USD. I got the impression that I won’t be able to build up feedback for trades under that amount. Is that correct?

Not exactly. You can build up reputation if the same user purchases a total of over 100 USD from you. So, for example, if you and user X have 4 trades: 5 USD, 50 USD, 20 USD, 30 USD, then the total trade volume between you will be above 100 USD and the feedback will be counted. Note, the USD amount is calculated based on the market value of the XMR when the trade is completed, not the actual USD price that you personally set on your ad.