I've had disappointing results when buying used skis, especially from eBay. Conversely, I've often found really good deals on new-old-stock skis from places like Level Nine Sports. If you can get neutral colors, everything you buy for your son will get double use.
For rank beginners, I suggest shoulder-height skis. Unless kids are very strong or heavy, I don't see much advantage to longer skis.
It looks like the Roces adjustable ski boots are taking over the market. I've never used them, but the idea sounds interesting. This is one area of equipment that can make or break the ski day. Comfort is king. Trying to get comfortable, but not sloppy, boots via the internet may be tricky.
You shouldn't need poles for those ages.
Just looking at the L9 site, you could outfit both kids for $450 or so. That should be a good yardstick when evaluating other deals.