For that level range, faerie apples (20) are sort of ok...not great, but you can sell them. logs are always pretty good across every level range. Corn and button mushrooms at 25 are pretty good. Crystals at 30 always have some demand. Noble grapes at 25 should sell well, but I haven't checked on it lately. Bear in mind tha tif I know these markets exist off the top of my head then so do thousands of other people. Sometimes they can become over saturated, though luckily it seems that there are a lot of people who just hate gathering.