1. If a person has made an offer to purchase and the seller is inclined to concluding the deal, then it is impermissible for anyone else to make an offer over the offer of the purchaser. This has been prohibited in the Hadith. However, if the seller is not inclined to the offer, then it is permissible for others to make an offer to purchase. An auction sale is excluded from this rule. Hence, it is permissible for many people to bid to purchase an item at the same time.
عن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: لا يسم المسلم على سوم أخيه. (صحيح مسلم، الرقم: 1515)
Hazrat Abu Hurairah (radhiyallahu anhu) reports that Hazrat Rasulullah (sallallahu alaihi wasallam) said, “A Muslim should not make an offer to purchase over the offer of his brother (i.e. in the case where the seller is inclined towards concluding the deal).”
عن ابن عمر رضي الله عنهما كان يقول: نهى النبي صلى الله عليه وسلم أن يبيع بعضكم على بيع بعض، ولا يخطب الرجل على خطبة أخيه، حتى يترك الخاطب قبله أو يأذن له الخاطب (صحيح البخاري، الرقم: 5142)
Hazrat Ibnu Umar (radhiyallahu anhuma) reports that Hazrat Rasulullah (sallallahu alaihi wasallam) said, “One should not place an offer to purchase something over the offer of his Muslim brother (i.e. in the case where the purchaser and seller are inclined towards concluding the sale), nor should he send a proposal over the proposal of his Muslim brother (when the both parties are inclined towards the nikaah), except if he retracts from the proposal himself, or gives him permission to do so.”