Just my opinion here...
#1 most important thing to remember DO NOT CARRY ANY JUNK KNIVES! None-zip-nada-EVER. Always carry good makers even if only the cheaper models they offer.
#2 Carry the 'popular' models. i.e. Spyderco-Delica, Endura, Police, Native. Benchmade-AFCK, TSEK, Griptillion. Offer to order anything they want but the proven sellers will sell, while the more exotic models will sit till just the right person comes along. Also be sure to carry both sizes if the models come in a 'large' and a 'small'. Many times new buyers don't realize just how large say a full size AFCK really is, but they will carry the 'mini' version.
#3 Offer a good selection of traditional slipjoints-Case, Old Timers, Hen and Roosters etc. Just a few models should do it here, unless you can get one of those display cases with like all twenty models.
#4 Absolutely Positively carry Swiss Army Knives, I'm partial to Victorinox but Wengers are in no way a 'bad product'. Carry many different models and have them in stock. These I would imagine would sell to all types of people and are definately not a niche item.
#5 Fixed Blades-off the top of my head? Buck, Old Timer, Benchmade, Spyderco, throw in a few KA-BAR\Camillus' for the traditionalists. Always have at least one Ontario Mil-Spec machete with sheath on hand!
#6 I would have one counter full of semi-exotic or specialized stuff, but again NO JUNK only quality stuff. This is for Benchmade Butterfly knives, big SOG folders, maybe an Al-Mar or two, perhaps even the local Custom guys stuff-give him some realestate. This is where you put the multi-tools (SOG's!!! are my favorites), neck knives, marlin-spike knives etc.
anyway....just some thoughts
Mongrel