In the Washington DC area, markets distingush between regular farmer's markets and "producer's markets." At a producer's market, you can only sell what you grow yourself. If it doesn't say "producer," then the all rules are off, and the vendor may have purchased the stuff at a produce auction. Check the website for your farmer's market; they are usually run by the county government. Or look on localharvest.org. They should tell you if it's a producer's market.
Blackberries and strawberries are very touchy anyway. Some varieties are sweeter than others. Blackberries can be notoriously sour.
And was this recently? Good gosh, it's been 94° F in Austin the past week. Berries are going to spoil very quickly no matter how good they are/were.
5'0" female, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.