30-38 is pretty much S, M, L, XL and XXL. Should be fine, in my opinion.
I feel like if you have a focus on designer stuff you'll stand out a bit, especially if you have a wider variety (for example, L4L was american based but only had 1:1 belts and shoes, and some glasses) you'll stand out and be the de-facto place to go to (assuming, again, that you have good customer service and your stock is actually high quality).
For kicks, get SMP+ only stuff and don't bother stocking anything under that, I think if you asked yourself "would I wear this outside, personally? (even if it's not your style, as long as it looks like the authentics)" would be a good way to go about it. SMP+ kicks that'd be interesting; Christian Louboutin, Giuseppe Zanotti, Martin Margiela, Rick Owens, Raf Simons, Saint Laurent, Balenciaga, Buscemi, etc are all gravy. I know David has most of these, but again if you want to get into kicks you should leave the Jordan/Nike/Adidas/etc market to others and focus on something specific. One of David's flaws with the 'designer' shoes he has is that he doesn't have a lot of pictures of the actual product on hand, or he has a lot of clutter and useless versions for no reason. If you can get most shoes up to size 12-13 too, that'd be killer.
Wallets (LV, Gucci, Versace, Hermes, etc.) Belts (LV, MCM, Gucci, Versace, Hermes, Fendi, Ferragamo were all 1:1 with L4L, so maybe try getting more than him? Givenchy, D&G, Goyard, etc) Glasses (Probably a small selection as I'm sure people would be interested. I don't fuck with glasses so I don't know much about 'em.) Polos (Ralph, Burberry, Lacoste, etc) General Clothing (A.P.C, ACNE, A WANG, Balmain, CDG, Helmut Lang, HBA, Givenchy, Kenzo, Krisvanassche, Visvim, Off-White, Raf, Rick, SLP, Versace, Pigalle, D&G, Y-3, Thom Browne, Yohji, etc are all brands that a carry a lot of weight with their names and people are interested in. No idea if most of these are even replicable, just throwing shit out there.)
I'm just blabbering now, but hey good luck. I'd much rather cop from an american based high quality rep place than pay 60$ shipping from china.