In abasing myself (εμαυτον ταπεινων). Humbling myself by making tents for a living while preaching in Corinth. He is ironical still about "doing a sin" (αμαρτιαν εποιησα).

For nought (δωρεαν). Gratis. Accusative of general reference, common adverb. It amounts to sarcasm to ask if he did a sin in preaching the gospel free of expense to them "that ye may be exalted."