Before posting an item to eBay, MAKE SURE YOU HAVE A SHIPPING BOX.  If not, you could spend many hours running around town looking for the correct sized box, or you could get a box that's too big and either waste time cutting it down or waste money on excess packing material / postage.

When selling a bulk lot, FILL THE BOX FIRST.  Put together a pile of similar items and place them in the box.  Whatever fits in that box, consider making that your bulk lot.  Be sure to leave room for safe packaging material, especially if the item is fragile or condition sensitive.

The USPS Flat Rate Priority Boxes work especially well for this.  There are two sizes and each will hold a surprising number of goodies.  At the time of this writingl, they will ship for $7.70 in the USA (plus 45 cents for delivery confirmation).  This rate is good for up to 70 POUNDS, and becomes cost effective for anything heavier than 2 pounds.  Even if you don't normally mail USPS, it's worthwhile to get some flat rate boxes and see what you can fit inside.

If you use Flat Rate Priority Boxes, keep a few similar size blank boxes, just in case the lot is purchased by an international bidder.  In some cases, shipping international can cost significantly more so it's a good idea to warn the bidders. Good luck!