starting batteries are meant to do just that.....start. They are not meant to have a load drawn from them for a long period of time, but are meant to give lots of amps for a short period of time. STARTING BATTERIES ARE THE WRONG TYPE OF BATTERIES TO RUN AN INVERTER.
Deep cycle batteries however ARE designed to have a load drawn for long periods. Most are rated for a certian number of amps over 8 hours. There is a limited number of times any battery can be cycled (discharged/recharged). Golf Cart batteries (or even better, Fork lift batteries) can be cycled many more times than a regualr starting battery and are some of the best batteries to use as "house" batteries to run the inverter or 12v loads in your rv or bus. golf cart batteries are far superior to most other deep cycle / rv batteries. Ideally for longevity you should never use more than half of the usefull energy stored in your batteries. Someoen listed earlier the "poop sheets by phred" that's an excellent source of information on this and many other RV topics.
Spend the money on new batteries. It would be most frustrating to have the nice shiney inverter that won't work because you wanted to save a few bucks and buy used batteries.
Quis custodiet ipsos custodes (who will watch the watchmen?)