Can I sell hotels I have contracted directly myself alongside the ones that come from suppliers?

Yes, most travel agency software lets you load your own directly contracted properties and show them in the same search results as the hotels that arrive from your connected suppliers, though the amount of detail you can hold about your own contracts depends on your plan.

In practice you set the property up once with its rooms, its board options, its photos and its description, then enter the rates and the availability you have agreed with the hotel, along with your cancellation terms and any stop sale dates. When a customer searches, your own property appears beside the supplier results and can be booked in the same basket, so your staff and your customers do not have to work in two different places. Because the contract is yours, the margin on those rooms is yours as well, and you are not adding a markup on top of somebody else's rate.

The honest limit is that a direct contract makes you responsible for the parts a supplier would normally handle. Nobody else is watching the allocation, so if you forget to close out rooms you have already sold, you can oversell and have to move a guest yourself. The confirmation goes to the hotel in whatever way you have agreed with them, and payment terms, deposits and release dates are between you and the property. Loading and maintaining the rates is manual work unless the hotel can send you a rate file.

Before you start, talk to one hotel you already work with, agree the rates, the release period and how they want to receive bookings, then load that single property and test a booking end to end so you know the process works before you add the rest.