Assuming a contract is assignable and discovering otherwise a week before closing is an avoidable and expensive mistake. It takes a minute to check.
Look for the assignment clause
Many standard forms are silent, some expressly permit assignment, and some prohibit it outright or require the seller's written consent.
If the contract is silent, assignment is generally permissible as a matter of ordinary contract principles, but silence is a weaker position than an express permission and it is worth improving.
Name the buyer in a way that allows it
"Buyer name and/or assigns" is the conventional formulation and it signals the intent from the outset. Some sellers will object; better to know that at the point of contract than later.
If you buy through an entity, understand that transferring interests in the entity is a different thing from assigning the contract, with different implications.
Sellers who will not permit it
Bank-owned property, many institutional sellers, HUD and a number of relocation companies prohibit assignment as a matter of policy. In those cases a double close is the usual route.
Do not assume you can quietly assign anyway. If the contract prohibits it, doing so is a breach and it is discoverable at settlement.
Tell the seller what you are doing
Sellers who understand the arrangement rarely object to it. Sellers who feel it was concealed frequently do, and a deal that falls apart at closing helps nobody.
Illinois has specific rules governing wholesaling activity, and how you present your role matters. This is a general explanation and not legal advice — if you transact regularly, have an attorney review your process and forms.