If you want to do it for your users, right in the moment you are going to give them the item, Schedule In and Out have no sense for you. We allows our customers to schedule the item they want to use so when they come to the helpdesk we only check out and in the item.
The reservation conflict message I suppose is this one:
Totally agree with you, WHD should allows you to filter those assets available for that date when creating a reservation for your customer.
Though, for customers doing the reservation themselves is not that difficult, because while scheduling they can see the calendar for that asset they want to reserve.