I'm in advice mode today. You have experienced the creeping favor. It starts out with a relatively small ask then snowballs from there. There is often an element of surprise or 3rd parties (in your case the buyer) who will be disappointed if you say no. Dealing with such people requires exceptional adult assertiveness, because they are being exceptionally demanding. You need to deny the additional request immediately, in whatever manner is best for all parties and your own style. Expect people to be annoyed at your refusal. If you can later, tell your cousin that such manipulative tactics wiil damage your relationship if they ever happen again. Also, expect to see this sort of behavior again anyway.