These are just suggestions which I think can help the overall experience for all buyers. Some are easy, some are difficult, but after making over 140 purchases here I think I have a pretty solid idea now of the kinds of improvements that will make shopping smoother, faster, less confusing, more accurate and offer the best value to avoid over-spending or missing out on needed parts.
All of this is in my imagination:
I start with no carts open, 0 parts in 0 carts.
I decide what my general shopping list will be (certain colors, certain parts or part types, etc). Then I start navigating through, I am able to easily pivot-switch between colors and part types without restarting my navigation or juggling multiple tabs. Meaning, it's 100% a filter-based search process that I can move forward with page-to-page without hitting the back button in my browser 100 times and without needing to juggle multiple tabs with multiple starting points.
Since we can rate stores based on multiple factors, I can easily hide or sort away stores I don't want to shop on or emphasize the ones I do. These ratings are specific to my preferences so they're only visible to me. Then, as I add parts to carts, I can denote how important the part is to me and what my count goal is. That accomplishes 2 things: It gives the site a chance to tell me "you've reached your goal" before I mindlessly keep adding the same part to multiple carts "just in case" as well as being able to rate my carts so that if I have multiple carts filled up I know which ones are the priority to buy first (nothing is more frustrating than building up a cart over 2-3 days only to find someone scooping up some of your more important part needs shortly before you planned to check out).
Also, in the shopping cart, the color of the part is VERY clear - because it's not dependent on a picture only, there is a box to the left of the part which is rendered with the precise (community-agreed-on) RGB value of the part. So that for the stores who fulfill based on printouts from BrickOwl (instead of a bin location system of their own) don't make color mistakes.
Then, after adding all the things I could possibly want to various store carts, the store cart view lets me further rate the carts based on a combination of: "desirable" part ratings, counts, lots, store rating, average price-per-part, number of times purchased from, etc, dispatch times, as well as sorting on any one of those - plus separating out the part cost vs shipping cost (if I see $10.50 vs $9.75 it doesn't give the real picture if $10.50 is $8.00 in parts and $2.50 in shipping and the other is $9.75 in parts and "shipping quote").
Sometimes, I go in and change the calculation of the importance of any rating filter - so that the sorting of buying preference is weighted to my particular interests. Finally, as I am checking out I am reminded of certain store policies like dispatch times during the checkout flow.
Additionally, the billing/shipping address directives are shown on ONE sub-selection instead of the confusing 2. That way the billing & shipping are both shown at the same time and the shipping one says "same as billing" + the additional options if needed. Also, the "notes to seller" is at or just after the shipping address selection because during checkout with PayPal sometimes it's shown as the pre-PayPal step and sometimes it's shown after so having it in a set place that doesn't change based on PayPal flow is great.
The rest of the shopping experience is perfectly fine so let's get to the post-delivery:
After receiving my delivery, I should be able to mark which parts/lots were wrong or not received, in poor condition, etc, in the completed cart - this gets added to the rating history as well as give BrickOwl/Lawrence and idea of which stores are on top of things vs which are not. Stores unable to meet a certain minimum level of quality will get warnings and if they don't shape up I don't think people should have them as a buying option without a warning like "this store is known to deliver incorrect or items of quality lower than marked - do you wish to proceed anyway?". These dings to the store are just private and not public but they are included in the overall store rating % and shown in messaging during browsing/shopping or checkout.
The above is totally in my imagination, but would help the shopping experience tremendously.